Integrated power generation and chemical production using solid oxide fuel cells

ABSTRACT

In various aspects, systems and methods are provided for operating a solid oxide fuel cell at conditions that can improve or optimize the combined electrical efficiency and chemical efficiency of the fuel cell. Instead of selecting conventional conditions for maximizing the electrical efficiency of a fuel cell, the operating conditions can allow for output of excess synthesis gas and/or hydrogen in the anode exhaust of the fuel cell. The synthesis gas and/or hydrogen can then be used in a variety of applications, including chemical synthesis processes and collection of hydrogen for use as a fuel.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of provisional U.S. Serial Nos.61/884,376, 61/884,545, 61/884,565, 61/884,586, 61/884,605, and61/884,635, all filed on Sep. 30, 2013, each of which is incorporated byreference herein in its entirety. This application further claims thebenefit of provisional U.S. Ser. No. 61/889,757, filed on Oct. 11, 2013,which is incorporated by reference herein in its entirety. Thisapplication further claims priority as continuations-in-part ofnon-provisional U.S. Ser. Nos. 14/197,391, 14/197,430, 14/197,551,14/197,613, 14/207,686, 14/207,687, 14/207,690, 14/207,691, 14/207,693,14/207,697, 14/207,698, 14/207,699, 14/207,706, 14/207,708, 14/207,710,14/207,711, 14/207,712, 14/207,714, 14/207,721, 14/207,726, and14/207,728, all filed on Mar. 13, 2014, and to U.S. Ser. Nos.14/315,419, 14/315,439, 14/315,479, 14/315,507, and 14/315,527, allfiled on Jun. 26, 2014, each of which is incorporated by referenceherein in its entirety.

This application is further related to two other co-pending U.S.applications, U.S. Ser. Nos. 14/486,200 and 14/486,177, filed on evendate herewith. Each of these co-pending U.S. applications is herebyincorporated by reference herein in its entirety.

FIELD OF THE INVENTION

In various aspects, the invention is related to chemical productionand/or power generation processes integrated with electrical powerproduction using solid oxide fuel cells.

BACKGROUND OF THE INVENTION

Solid oxide fuel cells utilize hydrogen and/or other fuels to generateelectricity. The hydrogen may be provided by reforming methane or otherreformable fuels in a steam reformer that is upstream of the fuel cellor within the fuel cell. Reformable fuels can encompasshydrocarbonaceous materials that can be reacted with steam and/or oxygenat elevated temperature and/or pressure to produce a gaseous productthat comprises hydrogen. Alternatively or additionally, fuel can bereformed in the anode cell in a solid oxide fuel cell, which can beoperated to create conditions that are suitable for reforming fuels inthe anode. Alternately or additionally, the reforming can occur bothexternally and internally to the fuel cell.

Traditionally, solid oxide fuel cells are operated to maximizeelectricity production per unit of fuel input, which may be referred toas the fuel cell's electrical efficiency. This maximization can be basedon the fuel cell alone or in a combined heat and power application. Inorder to achieve increased electrical production and to manage the heatgeneration, fuel utilization within a fuel cell is typically maintainedat 70% to 85%.

U.S. Patent Application Publication No. 2005/0123810 describes a systemand method for co-production of hydrogen and electrical energy. Theco-production system comprises a fuel cell and a separation unit, whichis configured to receive the anode exhaust stream and separate hydrogen.A portion of the anode exhaust is also recycled to the anode inlet. Theoperating ranges given in the '810 publication appear to be based on amolten carbonate fuel cell. Solid oxide fuel cells are described as analternative.

SUMMARY OF THE INVENTION

In an aspect, a method for producing electricity and hydrogen or syngasusing a solid oxide fuel cell having an anode and cathode is provided.The method introducing a fuel stream comprising a reformable fuel intothe anode of the solid oxide fuel cell, an internal reforming elementassociated with the anode of the solid oxide fuel cell, or a combinationthereof; introducing a cathode inlet stream comprising O₂ into thecathode of the solid oxide fuel cell; generating electricity within thesolid oxide fuel cell; and withdrawing, from an anode exhaust, a gasstream comprising H₂, a gas stream comprising H₂ and CO, or acombination thereof, wherein an electrical efficiency for the solidoxide fuel cell is between about 10% and about 50% and a total fuel cellproductivity for the solid oxide fuel cell is at least about 150 mW/cm².

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 schematically shows an example of a configuration for solid oxidefuel cells and associated reforming and separation stages.

FIG. 2 schematically shows another example of a configuration for solidoxide fuel cells and associated reforming and separation stages.

FIG. 3 schematically shows an example of the operation of a solid oxidefuel cell.

DETAILED DESCRIPTION OF THE EMBODIMENTS

Overview

In various aspects, systems and methods are provided that provide forthe production of large amounts of hydrogen or synthesis gas, inaddition to the production of electricity, from a solid oxide fuel cell(SOFC) at high total fuel cell efficiency. Aspects of the presentinvention can use either planar cells or tubular cells. The total fuelcell efficiency refers generally to the combined electrical efficiencyand chemical efficiency of the fuel cell. A fuller definition of totalfuel cell efficiency is provided subsequently.

Typical fuel cell systems can be designed and operated for optimizedelectrical efficiency at the expense of any other parameter(s). Heatproduced, whether in-situ and/or as the result of combusting off-gasesand anode products, can be employed to an extent needed to maintain fuelcell operations at steady conditions. As with most electricitygeneration methods, conventional fuel cell systems primarily value theelectrical product. Conventional fuel cell systems can be used inapplications where the primary purpose is the production of efficientelectrical power, such as in distributed generation or backupgeneration.

Aspects of the present invention can establish fuel cell operatingparameters to cause the total fuel cell efficiency to exceedconventional fuel cell efficiency. Additionally, or alternately, thepresent invention provides a method to increase the total fuel cellproductivity while maintaining very high overall system efficiency. Inone aspect, productivity is the total amount of useful products (e.g.,syngas, heat, electricity) produced per unit of time for a designatedamount of fuel cell capacity, for example as measured by the crosssectional area of a fuel cell. Instead of selecting conventionalconditions for maximizing the electrical efficiency of a fuel cell,operating conditions can produce much higher total fuel cell efficiencyand/or productivity for the overall system if the electrical efficiencyis allowed to fall below the optimal electrical efficiency sought in thetypical fuel cell systems described above. As described in more detailbelow, total fuel cell efficiency is a measure of the amount of energygenerated by a fuel cell relative to the amount of energy delivered tothe fuel cell, while productivity is a measure of the amount of energygenerated (the total chemical, electrical and heat energy) by a fuelcell relative to the size (such as anode area) of the fuel cell. Theconditions that can achieve high total fuel cell efficiency and/orproductivity can allow for output of excess synthesis gas and/orhydrogen in the anode exhaust of the fuel cell and can be achieved bycompletely or partially decoupling the inputs and outputs from the anodeand cathode so as to allow excess production of some products. Thisexcess can be enabled, for example, by decreasing the electricalefficiency of the cell (e.g., by operating at lower voltage) and/orusing the heat generated in-situ for efficient production of chemicalenergy (e.g., in the form of syngas). As a result, the fuel cell canprocess a much larger amount of total fuel input into the anode whilemaintaining a total output efficiency (the sum of chemical, electrical,and useful thermal energy) that is similar to or higher than what isknown in the art. The higher productivity allows for more efficient useof the fuel cell within the combined system.

The electrochemical processes occurring in the anode may result in ananode output flow of syngas containing at least a combination of H₂, CO,and CO₂. A water-gas shift reaction can then be used to generate adesired composition of synthesis gas and/or to increase or maximize H₂production relative to other syngas components. The synthesis gas and/orhydrogen can then be used in a variety of applications, including butnot limited to chemical synthesis processes and/or collection ofhydrogen for use as a “clean” fuel.

As used herein, the term “electrical efficiency” (“EE”) is defined asthe electrochemical power produced by the fuel cell divided by the rateof Lower Heating Value (“LHV”) of fuel input to the fuel cell. The fuelinputs to the fuel cell includes both fuel delivered to the anode aswell as any fuel used to maintain the temperature of the fuel cell, suchas fuel delivered to a burner associated with a fuel cell. In thisdescription, the power produced by the fuel may be described in terms ofLHV(el) fuel rate.

As used herein, the term “electrochemical power” or LHV(el) is the powergenerated by the circuit connecting the cathode to the anode in the fuelcell and the transfer of oxygen ions across the fuel cell's electrolyte.Electrochemical power excludes power produced or consumed by equipmentupstream or downstream from the fuel cell. For example, electricityproduced from heat in a fuel cell exhaust stream is not considered partof the electrochemical power. Similarly, power generated by a gasturbine or other equipment upstream of the fuel cell is not part of theelectrochemical power generated. The “electrochemical power” does nottake electrical power consumed during operation of the fuel cell intoaccount, or any loss incurred by conversion of the direct current toalternating current. In other words, electrical power used to supply thefuel cell operation or otherwise operate the fuel cell is not subtractedfrom the direct current power produced by the fuel cell. As used herein,the power density is the current density multiplied by voltage. As usedherein, the current density is the current per unit area. As usedherein, the total fuel cell power is the power density multiplied by thefuel cell area.

As used herein, the term “anode fuel input,” designated asLHV(anode_in), is the amount of fuel within the anode inlet stream. Theterm “fuel input”, designated as LHV(in), is the total amount of fueldelivered to the fuel cell, including both the amount of fuel within theanode inlet stream and the amount of fuel used to maintain thetemperature of the fuel cell. The fuel may include both reformable andnonreformable fuels, based on the definition of a reformable fuelprovided herein. Fuel input is not the same as fuel utilization.

As used herein, the term “total fuel cell efficiency” (“TFCE”) isdefined as: the electrochemical power generated by the fuel cell, plusthe rate of LHV of syngas produced by the fuel cell, divided by the rateof LHV of fuel input to the anode. In other words, TFCE=(LHV(el)+LHV(sgnet))/LHV(anode_in), where LHV(anode_in) refers to rate at which the LHVof the fuel components (such as H₂, CH₄, and/or CO) delivered to theanode and LHV(sg net) refers to a rate at which syngas (H₂, CO) isproduced in the anode, which is the difference between syngas input tothe anode and syngas output from the anode. LHV(el) describes theelectrochemical power generation of the fuel cell. The total fuel cellefficiency excludes heat generated by the fuel cell that is put tobeneficial use outside of the fuel cell. In operation, heat generated bythe fuel cell may be put to beneficial use by downstream equipment. Forexample, the heat may be used to generate additional electricity or toheat water. These uses, which occur apart from the fuel cell, are notpart of the total fuel cell efficiency, as the term is used in thisapplication. The total fuel cell efficiency is for the fuel celloperation only, and does not include power production, or consumption,upstream, or downstream, of the fuel cell.

As used herein, the term “chemical efficiency” is defined as the lowerheating value of H₂ and CO in the anode exhaust of the fuel cell, orLHV(sg out), divided by the fuel input, or LHV(in).

As used herein, the term “total fuel cell productivity” (“TFCP”) isdefined as the total energy value of products produced per unit ofcross-section fuel-cell area per unit of time due to the transformationof the input fuel. The fuel may be transformed in the oxidationreaction, the reformation reaction, and/or the water gas shift reaction.The total energy of the products may be expressed in any convenientunits, such as mW per cm². The products produced by the fuel cell caninclude electrochemical power, synthesis gas and/or hydrogen, and heat.The heat produced may be determined by measuring the temperaturedifference between the anode inlet and the anode outlet. As an example,the productivity of a fuel cell could be expressed as mW per cm² ofcross sectional area of the fuel cell anode. Fuel cell operatingconditions can optionally be selected to produce both a high total fuelcell productivity and a high total fuel cell efficiency.

As used herein, the term “total reformable fuel productivity” (“TRFP”)is the difference between the LHV of the reformable fuel input to theanode and the LHV of the reformable fuel received from the anode outletper unit² of the fuel cell's cross sectional area. The differencebetween the reformable fuel in the anode inlet and outlet can be aboutequal to the amount of the reformable fuel converted into synthesis gasand/or hydrogen minus the amount of newly produced synthesis gas and/orhydrogen that is consumed in the oxidation reaction that ultimatelyproduces electricity. Newly produced synthesis gas and/or hydrogen areproduced in the anode or in associated reforming stages heat integratedwith the fuel cell. Synthesis gas and/or hydrogen that are provided tothe anode inlet are not newly produced. Fuel cell operating conditionscan optionally be selected to produce both a high total reformable fuelproductivity and a high total fuel cell efficiency.

In some aspects, the operation of the fuel cells can be characterizedbased on electrical efficiency. Where fuel cells are operated to have alow electrical efficiency (EE), a solid oxide fuel cell can be operatedto have an electrical efficiency of about 50% or less, for example,about 45% EE or less, about 40% EE or less, about 35% EE or less, orabout 30% EE or less, about 25% EE or less, about 20% EE or less, about15% EE or less, or about 10% EE or less. Additionally or alternately,the EE can be at least about 5%, or at least about 10%, or at leastabout 15%, at least about 20%, at least about 25%, or at least about30%. Further additionally or alternately, the operation of the fuelcells can be characterized based on total fuel cell efficiency (TFCE),such as a combined electrical efficiency and chemical efficiency of thefuel cell(s). Where fuel cells are operated to have a high total fuelcell efficiency, a solid oxide fuel cell can be operated to have a TFCE(and/or combined electrical efficiency and chemical efficiency) of about55% or more, for example, about 60% or more, or about 65% or more, orabout 70% or more, or about 75% or more, or about 80% or more, or about85% or more. It is noted that, for a total fuel cell efficiency and/orcombined electrical efficiency and chemical efficiency, any additionalelectricity generated from use of excess heat generated by the fuel cellcan be excluded from the efficiency calculation.

In various aspects of the invention, the operation of the fuel cells canbe characterized based on a desired electrical efficiency of about 50%or less and a desired total fuel cell efficiency of 55% or more. Wherefuel cells are operated to have a desired electrical efficiency and adesired total fuel cell efficiency, a solid oxide fuel cell can beoperated to have an electrical efficiency of 50% or less with a TFCE ofabout 55% or more, for example, about 40% EE or less with a TFCE ofabout 60% or more, about 35% EE or less with a TFCE of about 65% ormore, about 30% EE or less with a TCFE of about 70% more, or about 20%EE or less with a TFCE of about 75% or more, or about 15% EE or lesswith a TFCE of about 80% or more, or about 10% EE or less with a TFCE ofabout 85% or more.

In various aspects of the invention, the operation of the fuel cells canbe characterized based on a desired total fuel cell productivity(“TFCP”) of about 150 mW/cm² or more and a desired total fuel cellefficiency of 55% or more. Where fuel cells are operated to have adesired TFCP above 150 mW/cm² and a desired total fuel cell efficiency,a solid oxide fuel cell can be operated to have a TFCE of about 55% ormore, for example, about 60% or more, about 65% or more, about 70% more,or about 75% or more, or about 80% or more, or about 85% or more. Whenfuel cells are operated to have a desired total fuel cell efficiency of55% or more, a solid oxide fuel cell can be operated to have a TFCP ofat least about 150 mW/cm², or at least about 200 mW/cm², or at leastabout 250 mW/cm², or at least about 300 mW/cm², or at least about 350mW/cm². In such aspects, the TFCP can be about 800 mW/cm² or less, orabout 700 mW/cm² or less or about 600 mW/cm² or less, or about 500mW/cm² or less, or about 400 mW/cm² or less.

In various aspects of the invention, the operation of the fuel cells canbe characterized based on a desired total reformable fuel productivityof about 75 mW/cm² or more and a desired total fuel cell efficiency of55% or more. Where fuel cells are operated to have a desired reformablefuel productivity above about 75 mW/cm² and a desired total fuel cellefficiency, a solid oxide fuel cell can be operated to have a TFCE ofabout 55% or more, for example, about 60% or more, about 65% or more,about 70% more, or about 75% or more, or about 80% or more, or about 85%or about 90% or more. When fuel cells are operated to have a desiredtotal fuel cell efficiency of 55% or more, a solid oxide fuel cell canbe operated to have a reformable fuel productivity of at least about 75mW/cm², or at least about 100 mW/cm², or at least about 125 mW/cm², orat least about 150 mW/cm², or at least about 175 mW/cm², or at leastabout 200 mW/cm² or at least about 300 mW/cm² In such aspects, thereformable fuel productivity can be about 600 mW/cm² or less, or about500 mW/cm² or less, or about 400 mW/cm² or less, or about 300 mW/cm² orless, or about 200 mW/cm² or less.

Operating a solid oxide fuel cell to have a desired electricalefficiency, chemical efficiency, and/or total fuel cell efficiency canbe achieved in various ways. In some aspects, the chemical efficiency ofa solid oxide fuel cell can be increased by increasing the amount ofreforming performed within the fuel cell (and/or within an associatedreforming stage in a fuel cell assembly) relative to the amount ofoxidation of hydrogen in the anode to generate electricity.Conventionally, solid oxide fuel cells have been operated to maximizethe efficiency of electrical power generation relative to the amount offuel consumed while maintaining a suitable heat balance to maintainoverall system temperatures. In this type of operating condition, fuelutilizations at the anode of about 70% to about 85% are desirable, inorder to maximize electrical efficiency at a desirable (i.e., high)voltage for the electric output and maintain heat balance within thefuel cell. At high fuel utilization values, only a modest amount ofhydrogen (or syngas) remains in the anode exhaust for formation ofsyngas. For example, at about 75% fuel utilization, about 25% of thefuel entering the anode can exit as a combination of syngas and/orunreacted fuel. The modest amount of hydrogen or syngas typically can beenough to maintain sufficient hydrogen concentrations at the anode tofacilitate the anode oxidation reaction and to provide enough fuel toheat reactants and/or inlet streams up to appropriate fuel celloperating temperatures.

In contrast to this conventional operation, a solid oxide fuel cell canbe operated at low fuel utilization and higher fuel flow rates, and withlittle or no recycle of fuel from the anode exhaust to the anode inlet.By operating at low fuel utilization while also reducing or minimizingrecycle of fuel to the anode inlet, a larger amount of H₂ and/or CO canbe available in the anode exhaust. This excess H₂ and CO can bewithdrawn as a syngas product and/or a hydrogen product. In variousaspects, the fuel utilization in the fuel cell can be at least about 5%,such as at least about 10% or at least about 15%, or at least about 20%.Additionally or alternately, in a low fuel utilization aspect, the fuelutilization can be about 60% or less, or about 50% or less, or about 40%or less.

One option for increasing the chemical efficiency of a fuel cell can beto increase the reformable hydrogen content of fuel delivered to thefuel cell. For example, the reformable hydrogen content of reformablefuel in the input stream delivered to the anode and/or to a reformingstage associated with the anode can be at least about 50% greater thanthe net amount of hydrogen reacted at the anode, such as at least about75% greater or at least about 100% greater. Additionally or alternately,the reformable hydrogen content of fuel in the input stream delivered tothe anode and/or to a reforming stage associated with the anode can beat least about 50% greater than the net amount of hydrogen reacted atthe anode, such as at least about 75% greater or at least about 100%greater. In various aspects, a ratio of the reformable hydrogen contentof the reformable fuel in the fuel stream relative to an amount ofhydrogen reacted in the anode can be at least about 1.5:1, or at leastabout 2.0:1, or at least about 2.5:1, or at least about 3.0:1.Additionally or alternately, the ratio of reformable hydrogen content ofthe reformable fuel in the fuel stream relative to the amount ofhydrogen reacted in the anode can be about 20:1 or less, such as about15:1 or less or about 10:1 or less. In one aspect, it is contemplatedthat less than 100% of the reformable hydrogen content in the anodeinlet stream can be converted to hydrogen. For example, at least about80% of the reformable hydrogen content in an anode inlet stream can beconverted to hydrogen in the anode and/or in an associated reformingstage(s), such as at least about 85%, or at least about 90%.

Either hydrogen or syngas can be withdrawn from the anode exhaust as achemical energy output. Hydrogen can be used as a clean fuel withoutgenerating greenhouse gases when it is burned or combusted.Additionally, hydrogen can be a valuable input for a variety of refineryprocesses and/or other synthesis processes. Syngas can also be avaluable input for a variety of processes. In addition to having fuelvalue, syngas can be used as a feedstock for producing other highervalue products, such as by using syngas as an input for Fischer-Tropschsynthesis and/or methanol synthesis processes.

In various aspects, the anode exhaust can have a ratio of H₂ to CO ofabout 1.5:1 to about 10:1, such as at least about 3.0:1, or at leastabout 4.0:1, or at least about 5.0:1, and/or about 8.0:1 or less orabout 6.0:1 or less. A syngas stream can be withdrawn from the anodeexhaust. In various aspects, a syngas stream withdrawn from an anodeexhaust can have a ratio of moles of H₂ to moles of CO of at least about0.9:1, such as at least about 1.0:1, or at least about 1.2:1, or atleast about 1.5:1, or at least about 1.7:1, or at least about 1.8:1, orat least about 1.9:1. Additionally or alternately, the molar ratio of H₂to CO in a syngas withdrawn from an anode exhaust can be about 3.0:1 orless, such as about 2.7:1 or less, or about 2.5:1 or less, or about2.3:1 or less, or about 2.2:1 or less, or about 2.1:1 or less. However,many types of syngas applications benefit from syngas with a molar ratioof H₂ to CO of at least about 1.5:1 to about 2.5:1 or less, so forming asyngas stream with a molar ratio of H₂ to CO content of, for example,about 1.7:1 to about 2.3:1 may be desirable for some applications.

Syngas can be withdrawn from an anode exhaust by any convenient method.In some aspects, syngas can be withdrawn from the anode exhaust byperforming separations on the anode exhaust to remove at least a portionof the components in the anode exhaust that are different from H₂ andCO. For example, an anode exhaust can first be passed through anoptional water-gas shift stage to adjust the relative amounts of H₂ andCO. One or more separation stages can then be used to remove H₂O and/orCO₂ from the anode exhaust. The remaining portion of the anode exhaustcan then correspond to the syngas stream, which can then be withdrawnfor use in any convenient manner. Additionally or alternately, thewithdrawn syngas stream can be passed through one or more water-gasshift stages and/or passed through one or more separation stages.

It is noted that an additional or alternative way of modifying the molarratio of H₂ to CO in the withdrawn syngas can be to separate an H₂stream from the anode exhaust and/or the syngas, such as by performing amembrane separation. Such a separation to form a separate H₂ outputstream can be performed at any convenient location, such as prior toand/or after passing the anode exhaust through a water-gas shiftreaction stage, and prior to and/or after passing the anode exhaustthrough one or more separation stages for removing components in theanode exhaust different from H₂ and CO. Optionally, a water-gas shiftstage can be used both before and after separation of an H₂ stream fromthe anode exhaust. In an additional or alternative embodiment, H₂ canoptionally be separated from the withdrawn syngas stream. In someaspects, a separated H₂ stream can correspond to a high purity H₂stream, such as an H₂ stream containing at least about 90 vol % of H₂,such as at least about 95 vol % of H₂ or at least about 99 vol % of H₂.

As an addition, complement, and/or alternative to the fuel celloperating strategies described herein, a solid oxide fuel cell (such asa fuel cell assembly) can be operated with an excess of reformable fuelrelative to the amount of hydrogen reacted in the anode of the fuelcell. Instead of selecting the operating conditions of a fuel cell toimprove or maximize the electrical efficiency of the fuel cell, anexcess of reformable fuel can be passed into the anode of the fuel cellto increase the chemical energy output of the fuel cell. Optionally butpreferably, this can lead to an increase in the total efficiency of thefuel cell based on the combined electrical efficiency and chemicalefficiency of the fuel cell.

In some aspects, the reformable hydrogen content of reformable fuel inthe input stream delivered to the anode and/or to a reforming stageassociated with the anode can be at least about 50% greater than theamount of hydrogen oxidized in the anode, such as at least about 75%greater or at least about 100% greater. In various aspects, a ratio ofthe reformable hydrogen content of the reformable fuel in the fuelstream relative to an amount of hydrogen reacted in the anode can be atleast about 1.5:1, or at least about 2.0:1, or at least about 2.5:1, orat least about 3.0:1. Additionally or alternately, the ratio ofreformable hydrogen content of the reformable fuel in the fuel streamrelative to the amount of hydrogen reacted in the anode can be about20:1 or less, such as about 15:1 or less or about 10:1 or less. In oneaspect, it is contemplated that less than 100% of the reformablehydrogen content in the anode inlet stream can be converted to hydrogen.For example, at least about 80% of the reformable hydrogen content in ananode inlet stream can be converted to hydrogen in the anode and/or inan associated reforming stage, such as at least about 85%, or at leastabout 90%.

Additionally or alternately, the amount of reformable fuel delivered tothe anode can be characterized based on the Lower Heating Value (LHV) ofthe reformable fuel relative to the LHV of the hydrogen oxidized in theanode. This can be referred to as a reformable fuel surplus ratio. Insuch an alternative, the reformable fuel surplus ratio can be at leastabout 2.0, such as at least about 2.5, or at least about 3.0, or atleast about 4.0. Additionally or alternately, the reformable fuelsurplus ratio can be about 25.0 or less, such as about 20.0 or less, orabout 15.0 or less, or about 10.0 or less.

In various aspects of the invention, the operation of the fuel cells canbe characterized based on a desired total fuel cell productivity(“TFCP”) of at least about 150 mW/cm² and a desired reformable fuelsurplus ratio. For example, a solid oxide fuel cell can be operated tohave a TFCP of at least about 150 mW/cm² and a reformable fuel surplusratio of at least about 2.0, such as at least about 2.5, or at leastabout 3.0, or at least about 4.0. Additionally or alternately, the TFCPcan be above about 150 mW/cm² and the reformable fuel surplus ratio canbe about 25.0 or less, such as about 20.0 or less, or about 15.0 orless, or about 10.0 or less. When fuel cells are operated to have areformable fuel surplus ratio of at least about 2.0, a solid oxide fuelcell can be operated to have a TFCP of at least about 150 mW/cm², or atleast about 200 mW/cm², or at least about 250 mW/cm², or at least about300 mW/cm², or at least about 350 mW/cm². In such aspects, the TFCP canbe about 800 mW/cm² or less, or about 700 mW/cm² or less or about 600mW/cm² or less, or about 500 mW/cm² or less, or about 400 mW/cm² orless.

In various aspects of the invention, the operation of the fuel cells canbe characterized based on a desired total reformable fuel productivityof about 75 mW/cm² or more and a desired a reformable fuel surplusratio. In some aspects, fuel cells can be operated to have a desiredreformable fuel productivity above about 75 mW/cm² and a reformable fuelsurplus ratio of at least about 2.0, such as at least about 2.5, or atleast about 3.0, or at least about 4.0. Additionally or alternately, thetotal reformable fuel productivity can be above about 75 mW/cm² and thereformable fuel surplus ratio can be about 25.0 or less, such as about20.0 or less, or about 15.0 or less, or about 10.0 or less. When fuelcells are operated to have a reformable fuel surplus ratio of at leastabout 2.0, a solid oxide fuel cell can be operated to have a reformablefuel productivity of at least about 75 mW/cm², or at least about 100mW/cm², or at least about 125 mW/cm², or at least about 150 mW/cm², orat least about 175 mW/cm², or at least about 200 mW/cm² or at leastabout 300 mW/cm² In such aspects, the reformable fuel productivity canbe about 600 mW/cm² or less, or about 500 mW/cm² or less, or about 400mW/cm² or less, or about 300 mW/cm² or less, or about 200 mW/cm² orless.

As an addition, complement, and/or alternative to the fuel celloperating strategies described herein, a solid oxide fuel cell can beoperated so that the amount of reforming can be selected relative to theamount of oxidation in order to achieve a desired thermal ratio for thefuel cell. As used herein, the “thermal ratio” is defined as the heatproduced by exothermic reactions in a fuel cell assembly divided by theendothermic heat demand of reforming reactions occurring within the fuelcell assembly. Expressed mathematically, the thermal ratio(TH)=Q_(EX)/Q_(EN), where Q_(EX) is the sum of heat produced byexothermic reactions and Q_(EN) is the sum of heat consumed by theendothermic reactions occurring within the fuel cell. Note that the heatproduced by the exothermic reactions corresponds to any heat due toreforming reactions, water gas shift reactions, and the electrochemicalreactions in the cell. The heat generated by the electrochemicalreactions can be calculated based on the ideal electrochemical potentialof the fuel cell reaction across the electrolyte minus the actual outputvoltage of the fuel cell. For example, the ideal electrochemicalpotential of the reaction in a SOFC is believed to be about 1.04V basedon the net reaction that occurs in the cell. During operation of theSOFC, the cell will typically have an output voltage less than 1.1 V dueto various losses. For example, a common output/operating voltage can beabout 0.65 V, or about 0.7 V, or about 0.75 V, or about 0.8V. The heatgenerated is equal to the electrochemical potential of the cell (e.g.,˜1.04V) minus the operating voltage. For example, the heat produced bythe electrochemical reactions in the cell is ˜0.34 V when the outputvoltage of ˜0.7V. Thus, in this scenario, the electrochemical reactionswould produce ˜0.7 V of electricity and ˜0.34 V of heat energy. In suchan example, the ˜0.7 V of electrical energy is not included as part ofQ_(EX). In other words, heat energy is not electrical energy.

In various aspects, the operating parameters of the SOFC can be set toachieve an operating voltage below at least 0.7 V, such as at leastbelow 0.65 V, or such as at least below 0.6 V, or such as at least below0.5V, or such as at least below 0.4 V, or such as at least below 0.3 V.

In various aspects, a thermal ratio can be determined for any convenientfuel cell structure, such as a fuel cell stack, an individual fuel cellwithin a fuel cell stack, a fuel cell stack with an integrated reformingstage, a fuel cell stack with an integrated endothermic reaction stage,or a combination thereof. The thermal ratio may also be calculated fordifferent units within a fuel cell stack, such as an assembly of fuelcells or fuel cell stacks. For example, the thermal ratio may becalculated for a single anode within a single fuel cell, an anodesection within a fuel cell stack, or an anode section within a fuel cellstack along with integrated reforming stages and/or integratedendothermic reaction stage elements in sufficiently close proximity tothe anode section to be integrated from a heat integration standpoint.As used herein, “an anode section” comprises anodes within a fuel cellstack that share a common inlet or outlet manifold.

In various aspects of the invention, the operation of the fuel cells canbe characterized based on a thermal ratio. Where fuel cells are operatedto have a desired thermal ratio, a solid oxide fuel cell can be operatedto have a thermal ratio of about 1.5 or less, for example about 1.3 orless, or about 1.15 or less, or about 1.0 or less, or about 0.95 orless, or about 0.90 or less, or about 0.85 or less, or about 0.80 orless, or about 0.75 or less. Additionally or alternately, the thermalratio can be at least about 0.25, or at least about 0.35, or at leastabout 0.45, or at least about 0.50.

In various aspects of the invention, the operation of the fuel cells canbe characterized based on a desired total reformable fuel productivityof about 75 mW/cm² or more and a desired thermal ratio. In some aspects,fuel cells can be operated to have a desired reformable fuelproductivity above about 75 mW/cm² and a thermal ratio of about 1.5 orless, for example about 1.3 or less, or about 1.15 or less, or about 1.0or less, or about 0.95 or less, or about 0.90 or less, or about 0.85 orless, or about 0.80 or less, or about 0.75 or less. Additionally oralternately, the total reformable fuel productivity can be above about75 mW/cm² and the thermal ratio can be at least about 0.25, or at leastabout 0.35, or at least about 0.45, or at least about 0.50. When fuelcells are operated to have a thermal ratio of about 0.25 to about 1.3, asolid oxide fuel cell can be operated to have a reformable fuelproductivity of at least about 75 mW/cm², or at least about 100 mW/cm²,or at least about 125 mW/cm², or at least about 150 mW/cm², or at leastabout 175 mW/cm², or at least about 200 mW/cm² or at least about 300mW/cm² In such aspects, the reformable fuel productivity can be about600 mW/cm² or less, or about 500 mW/cm² or less, or about 400 mW/cm² orless, or about 300 mW/cm² or less, or about 200 mW/cm² or less.

In various aspects of the invention, the operation of the fuel cells canbe characterized based on a desired total fuel cell productivity ofabout 150 mW/cm² or more and a desired a thermal ratio. In one aspects,fuel cells are operated to have a desired total fuel cell productivityabove about 150 mW/cm² and a thermal ratio of about 1.5 or less, forexample about 1.3 or less, or about 1.15 or less, or about 1.0 or less,or about 0.95 or less, or about 0.90 or less, or about 0.85 or less, orabout 0.80 or less, or about 0.75 or less. Additionally or alternately,the total fuel cell productivity can be above about 150 mW/cm² and thethermal ratio can be at least about 0.25, or at least about 0.35, or atleast about 0.45, or at least about 0.50. When fuel cells are operatedto have a thermal ratio of about 0.25 to about 1.3, a solid oxide fuelcell can be operated to have a TFCP of at least about 150 mW/cm², or atleast about 200 mW/cm², or at least about 250 mW/cm², or at least about300 mW/cm², or at least about 350 mW/cm². In such aspects, the TFCP canbe about 800 mW/cm² or less, or about 700 mW/cm² or less or about 600mW/cm² or less, or about 500 mW/cm² or less, or about 400 mW/cm² orless.

Additionally or alternately, in some aspects the fuel cell can beoperated to have a temperature rise between anode input and anode outputof about 40° C. or less, such as about 20° C. or less, or about 10° C.or less. Further additionally or alternately, the fuel cell can beoperated to have an anode outlet temperature that is from about 10° C.lower to about 10° C. higher than the temperature of the anode inlet.Still further additionally or alternately, the fuel cell can be operatedto have an anode inlet temperature that is greater than the anode outlettemperature, such as at least about 5° C. greater, or at least about 10°C. greater, or at least about 20° C. greater, or at least about 25° C.greater. Yet still further additionally or alternately, the fuel cellcan be operated to have an anode inlet temperature that is greater thanthe anode outlet temperature by about 100° C. or less, such as by about80° C. or less, or about 60° C. or less, or about 50° C. or less, orabout 40° C. or less, or about 30° C. or less, or about 20° C. or less,or about 10° C. or less. Minimizing the difference between the anodeinlet temperature and outlet temperature may help maintain mechanicalintegrity of the ceramic components in the solid oxide fuel cell.

As an addition, complement, and/or alternative to the fuel celloperating strategies described herein, a solid oxide fuel cell (such asa fuel cell assembly) can be operated at conditions that can provideincreased power density. The power density of a fuel cell corresponds tothe actual operating voltage V_(A) multiplied by the current density I.For a solid oxide fuel cell operating at a voltage V_(A), the fuel cellalso can tend to generate waste heat, the waste heat defined as(V₀−V_(A))*I based on the differential between V_(A) and the idealvoltage V₀ for a fuel cell providing current density I. A portion ofthis waste heat can be consumed by reforming of a reformable fuel withinthe anode of the fuel cell. The remaining portion of this waste heat canbe absorbed by the surrounding fuel cell structures and gas flows,resulting in a temperature differential across the fuel cell. Underconventional operating conditions, the power density of a fuel cell canbe limited based on the amount of waste heat that the fuel cell cantolerate without compromising the integrity of the fuel cell.

In various aspects, the amount of waste heat that a fuel cell cantolerate can be increased by performing an effective amount of anendothermic reaction within the fuel cell. One example of an endothermicreaction includes steam reforming of a reformable fuel within a fuelcell anode and/or in an associated reforming stage, such as anintegrated reforming stage in a fuel cell stack. By providing additionalreformable fuel to the anode of the fuel cell (or to anintegrated/associated reforming stage), additional reforming can beperformed so that additional waste heat can be consumed. This can reducethe amount of temperature differential across the fuel cell, thusallowing the fuel cell to operate under an operating condition with anincreased amount of waste heat. The loss of electrical efficiency can beoffset by the creation of an additional product stream, such as syngasand/or H₂, that can be used for various purposes including additionalelectricity generation further expanding the power range of the system.

In various aspects, the amount of waste heat generated by a fuel cell,(V₀−V_(A))*I as defined above, can be at least about 30 mW/cm², such asat least about 40 mW/cm², or at least about 50 mW/cm², or at least about60 mW/cm², or at least about 70 mW/cm², or at least about 80 mW/cm², orat least about 100 mW/cm², or at least about 120 mW/cm², or at leastabout 140 mW/cm², or at least about 160 mW/cm², or at least about 180mW/cm², or at least about 200 mW/cm², or at least about 220 mW/cm², orat least about 250 mW/cm², or at least about 300 mW/cm² Additionally oralternately, the amount of waste heat generated by a fuel cell can beless than about 400 mW/cm², such as less than about 300 mW/cm², or lessthan about 200 mW/cm², or less than about 175 mW/cm², or less than about150 mW/cm².

Although the amount of waste heat being generated can be relativelyhigh, such waste heat may not necessarily represent operating a fuelcell with poor efficiency. Instead, the waste heat can be generated dueto operating a fuel cell at an increased power density. Part ofimproving the power density of a fuel cell can include operating thefuel cell at a sufficiently high current density. In various aspects,the current density generated by the fuel cell can be at least about 150mA/cm², such as at least about 160 mA/cm², or at least about 170 mA/cm²,or at least about 180 mA/cm², or at least about 190 mA/cm², or at leastabout 200 mA/cm², or at least about 300 mA/cm², or at least about 400mA/cm², or at least about 800 mA/cm². Additionally or alternately, thecurrent density generated by the fuel cell can be about 800 mA/cm² orless, such as 450 mA/cm², or less, or 300 mA/cm², or less or 250 mA/cm²,or less or 200 mA/cm² or less.

In various aspects, to allow a fuel cell to be operated with increasedpower generation and increased generation of waste heat, an effectiveamount of an endothermic reaction (such as a reforming reaction) can beperformed. Alternatively, other endothermic reactions unrelated to anodeoperations can be used to utilize the waste heat by interspersing“plates” or stages into the fuel cell array in thermal communication butnot in fluid communication with either anodes or cathodes. The effectiveamount of the endothermic reaction can be performed in an associatedreforming stage, an integrated reforming stage, an integrated stackelement for performing an endothermic reaction, or a combinationthereof. The effective amount of the endothermic reaction can correspondto an amount sufficient to reduce the temperature rise from the fuelcell inlet to the fuel cell outlet to about 100° C. or less, such asabout 90° C. or less, or about 80° C. or less, or about 70° C. or less,or about 60° C. or less, or about 50° C. or less, or about 40° C. orless, or about 30° C. or less. Additionally or alternately, theeffective amount of the endothermic reaction can correspond to an amountsufficient to cause a temperature decrease from the fuel cell inlet tothe fuel cell outlet of about 100° C. or less, such as about 90° C. orless, or about 80° C. or less, or about 70° C. or less, or about 60° C.or less, or about 50° C. or less, or about 40° C. or less, or about 30°C. or less, or about 20° C. or less, or about 10° C. or less. Atemperature decrease from the fuel cell inlet to the fuel cell outletcan occur when the effective amount of the endothermic reaction exceedsthe waste heat generated. Additionally or alternately, this cancorrespond to having the endothermic reaction(s) (such as a combinationof reforming and another endothermic reaction) consume at least about40% of the waste heat generated by the fuel cell, such as consuming atleast about 50% of the waste heat, or at least about 60% of the wasteheat, or at least about 75% of the waste heat. Further additionally oralternately, the endothermic reaction(s) can consume about 95% of thewaste heat or less, such as about 90% of the waste heat or less, orabout 85% of the waste heat or less.

Additional Definitions

Syngas: In this description, syngas is defined as mixture of H₂ and COin any ratio. Optionally, H₂O and/or CO₂ may be present in the syngas.Optionally, inert compounds (such as nitrogen) and residual reformablefuel compounds may be present in the syngas. If components other than H₂and CO are present in the syngas, the combined volume percentage of H₂and CO in the syngas can be at least 25 vol % relative to the totalvolume of the syngas, such as at least 40 vol %, or at least 50 vol %,or at least 60 vol %. Additionally or alternately, the combined volumepercentage of H₂ and CO in the syngas can be 100 vol % or less, such as95 vol % or less or 90 vol % or less.

Reformable fuel: A reformable fuel is defined as a fuel that containscarbon-hydrogen bonds that can be reformed to generate H₂. Hydrocarbonsare examples of reformable fuels, as are other hydrocarbonaceouscompounds such as alcohols. Although CO and H₂O can participate in awater gas shift reaction to form hydrogen, CO is not considered areformable fuel under this definition.

Reformable hydrogen content: The reformable hydrogen content of a fuelis defined as the number of H₂ molecules that can be derived from a fuelby reforming the fuel and then driving the water gas shift reaction tocompletion to maximize H₂ production. It is noted that H₂ by definitionhas a reformable hydrogen content of 1, although H₂ itself is notdefined as a reformable fuel herein. Similarly, CO has a reformablehydrogen content of 1. Although CO is not strictly reformable, drivingthe water gas shift reaction to completion will result in exchange of aCO for an H₂. As examples of reformable hydrogen content for reformablefuels, the reformable hydrogen content of methane is 4 H₂ moleculeswhile the reformable hydrogen content of ethane is 7 H₂ molecules. Moregenerally, if a fuel has the composition CxHyOz, then the reformablehydrogen content of the fuel at 100% reforming and water-gas shift isn(H₂ max reforming)=2x+y/2−z. Based on this definition, fuel utilizationwithin a cell can then be expressed as n(H₂ ox)/n(H₂ max reforming). Ofcourse, the reformable hydrogen content of a mixture of components canbe determined based on the reformable hydrogen content of the individualcomponents. The reformable hydrogen content of compounds that containother heteroatoms, such as oxygen, sulfur or nitrogen, can also becalculated in a similar manner.

Oxidation Reaction: In this discussion, the oxidation reaction withinthe anode of a fuel cell is defined as the reaction corresponding tooxidation of H₂ by reaction with O²⁻ to form H₂O. It is noted that thereforming reaction within the anode, where a compound containing acarbon-hydrogen bond is converted into H₂ and CO or CO₂, is excludedfrom this definition of the oxidation reaction in the anode. Thewater-gas shift reaction is similarly outside of this definition of theoxidation reaction. It is further noted that references to a combustionreaction are defined as references to reactions where H₂ or a compoundcontaining carbon-hydrogen bond(s) are reacted with O₂ to form H₂O andcarbon oxides in a non-electrochemical burner, such as the combustionzone of a combustion-powered generator.

Aspects of the invention can adjust anode fuel parameters to achieve adesired operating range for the fuel cell. Anode fuel parameters can becharacterized directly, and/or in relation to other fuel cell processesin the form of one or more ratios. For example, the anode fuelparameters can be controlled to achieve one or more ratios including afuel utilization, a fuel cell heating value utilization, a fuel surplusratio, a reformable fuel surplus ratio, a reformable hydrogen contentfuel ratio, and combinations thereof.

Fuel utilization: Fuel utilization is an option for characterizingoperation of the anode based on the amount of oxidized fuel relative tothe reformable hydrogen content of an input stream can be used to definea fuel utilization for a fuel cell. In this discussion, “fuelutilization” is defined as the ratio of the amount of hydrogen oxidizedin the anode for production of electricity (as described above) versusthe reformable hydrogen content of the anode input (including anyassociated reforming stages). Reformable hydrogen content has beendefined above as the number of H₂ molecules that can be derived from afuel by reforming the fuel and then driving the water gas shift reactionto completion to maximize H₂ production. For example, each methaneintroduced into an anode and exposed to steam reforming conditionsresults in generation of the equivalent of 4 H₂ molecules at maxproduction. (Depending on the reforming and/or anode conditions, thereforming product can correspond to a non-water gas shifted product,where one or more of the H₂ molecules is present instead in the form ofa CO molecule.) Thus, methane is defined as having a reformable hydrogencontent of 4 H₂ molecules. As another example, under this definitionethane has a reformable hydrogen content of 7 H₂ molecules.

The utilization of fuel in the anode can also be characterized bydefining a heating value utilization based on a ratio of the LowerHeating Value of hydrogen oxidized in the anode due to the fuel cellanode reaction relative to the Lower Heating Value of all fuel deliveredto the anode and/or a reforming stage associated with the anode. The“fuel cell heating value utilization” as used herein can be computedusing the flow rates and Lower Heating Value (LHV) of the fuelcomponents entering and leaving the fuel cell anode. As such, fuel cellheating value utilization can be computed as(LHV(anode_in)−LHV(anode_out))/LHV(anode_in), where LHV(anode_in) andLHV(anode_out) refer to the LHV of the fuel components (such as H₂, CH₄,and/or CO) in the anode inlet and outlet streams or flows, respectively.In this definition, the LHV of a stream or flow may be computed as a sumof values for each fuel component in the input and/or output stream. Thecontribution of each fuel component to the sum can correspond to thefuel component's flow rate (e.g., mol/hr) multiplied by the fuelcomponent's LHV (e.g., joules/mol).

Lower Heating Value: The lower heating value is defined as the enthalpyof combustion of a fuel component to vapor phase, fully oxidizedproducts (e.g., vapor phase CO₂ and H₂O product). For example, any CO₂present in an anode input stream does not contribute to the fuel contentof the anode input, since CO₂ is already fully oxidized. For thisdefinition, the amount of oxidation occurring in the anode due to theanode fuel cell reaction is defined as oxidation of H₂ in the anode aspart of the electrochemical reaction in the anode, as defined above.

It is noted that, for the special case where the only fuel in the anodeinput flow is H₂, the only reaction involving a fuel component that cantake place in the anode represents the conversion of H₂ into H₂O. Inthis special case, the fuel utilization simplifies to (H₂-rate-in minusH₂-rate-out)/H₂-rate-in. In such a case, H₂ would be the only fuelcomponent, and so the H₂ LHV would cancel out of the equation. In themore general case, the anode feed may contain, for example, CH₄, H₂, andCO in various amounts. Because these species can typically be present indifferent amounts in the anode outlet, the summation as described abovecan be needed to determine the fuel utilization.

Alternatively or in addition to fuel utilization, the utilization forother reactants in the fuel cell can be characterized. For example, theoperation of a fuel cell can additionally or alternately becharacterized with regard to “oxidant” utilization. The values foroxidant utilization can be specified in a similar manner.

Fuel surplus ratio: Still another way to characterize the reactions in asolid oxide fuel cell is by defining a utilization based on a ratio ofthe Lower Heating Value of all fuel delivered to the anode and/or areforming stage associated with the anode relative to the Lower HeatingValue of hydrogen oxidized in the anode due to the fuel cell anodereaction. This quantity will be referred to as a fuel surplus ratio. Assuch the fuel surplus ratio can be computed as (LHV(anode_in)/(LHV(anode_in)−LHV(anode_out)) where LHV(anode_in) andLHV(anode_out) refer to the LHV of the fuel components (such as H₂, CH₄,and/or CO) in the anode inlet and outlet streams or flows, respectively.In various aspects of the invention, a solid oxide fuel cell can beoperated to have a fuel surplus ratio of at least about 1.0, such as atleast about 1.5, or at least about 2.0, or at least about 2.5, or atleast about 3.0, or at least about 4.0. Additionally or alternately, thefuel surplus ratio can be about 25.0 or less.

It is noted that not all of the reformable fuel in the input stream forthe anode may be reformed. Preferably, at least about 90% of thereformable fuel in the input stream to the anode (and/or into anassociated reforming stage) can be reformed prior to exiting the anode,such as at least about 95% or at least about 98%. In some alternativeaspects, the amount of reformable fuel that is reformed can be fromabout 75% to about 90%, such as at least about 80%.

The above definition for fuel surplus ratio provides a method forcharacterizing the amount of reforming occurring within the anode and/orreforming stage(s) associated with a fuel cell relative to the amount offuel consumed in the fuel cell anode for generation of electric power.

Optionally, the fuel surplus ratio can be modified to account forsituations where fuel is recycled from the anode output to the anodeinput. When fuel (such as H₂, CO, and/or unreformed or partiallyreformed hydrocarbons) is recycled from anode output to anode input,such recycled fuel components do not represent a surplus amount ofreformable or reformed fuel that can be used for other purposes.Instead, such recycled fuel components merely indicate a desire toreduce fuel utilization in a fuel cell.

Reformable fuel surplus ratio: Calculating a reformable fuel surplusratio is one option to account for such recycled fuel components is tonarrow the definition of surplus fuel, so that only the LHV ofreformable fuels is included in the input stream to the anode. As usedherein the “reformable fuel surplus ratio” is defined as the LowerHeating Value of reformable fuel delivered to the anode and/or areforming stage associated with the anode relative to the Lower HeatingValue of hydrogen oxidized in the anode due to the fuel cell anodereaction. Under the definition for reformable fuel surplus ratio, theLHV of any H₂ or CO in the anode input is excluded. Such an LHV ofreformable fuel can still be measured by characterizing the actualcomposition entering a fuel cell anode, so no distinction betweenrecycled components and fresh components needs to be made. Although somenon-reformed or partially reformed fuel may also be recycled, in mostaspects the majority of the fuel recycled to the anode can correspond toreformed products such as H₂ or CO. Expressed mathematically, thereformable fuel surplus ratio (R_(RFS))=LHV_(RF)/LHV_(OH), whereLHV_(RF) is the Lower Heating Value (LHV) of the reformable fuel andLHV_(OH) is the Lower Heating Value (LHV) of the hydrogen oxidized inthe anode. The LHV of the hydrogen oxidized in the anode may becalculated by subtracting the LHV of the anode outlet stream from theLHV of the anode inlet stream (e.g., LHV(anode_in)−LHV(anode_out)). Invarious aspects of the invention, a solid oxide fuel cell can beoperated to have a reformable fuel surplus ratio of at least about 0.25,such as at least about 0.5, or at least about 1.0, or at least about1.5, or at least about 2.0, or at least about 2.5, or at least about3.0, or at least about 4.0. Additionally or alternately, the reformablefuel surplus ratio can be about 25.0 or less. It is noted that thisnarrower definition based on the amount of reformable fuel delivered tothe anode relative to the amount of oxidation in the anode candistinguish between two types of fuel cell operation methods that havelow fuel utilization. Some fuel cells achieve low fuel utilization byrecycling a substantial portion of the anode output back to the anodeinput. This recycle can allow any hydrogen in the anode input to be usedagain as an input to the anode. This can reduce the amount of reforming,as even though the fuel utilization is low for a single pass through thefuel cell, at least a portion of the unused fuel is recycled for use ina later pass. Thus, fuel cells with a wide variety of fuel utilizationvalues may have the same ratio of reformable fuel delivered to the anodereforming stage(s) versus hydrogen oxidized in the anode reaction. Inorder to change the ratio of reformable fuel delivered to the anodereforming stages relative to the amount of oxidation in the anode,either an anode feed with a native content of non-reformable fuel needsto be identified, or unused fuel in the anode output needs to bewithdrawn for other uses, or both.

Reformable hydrogen surplus ratio: Still another option forcharacterizing the operation of a fuel cell is based on a “reformablehydrogen surplus ratio.” The reformable fuel surplus ratio defined aboveis defined based on the lower heating value of reformable fuelcomponents. The reformable hydrogen surplus ratio is defined as thereformable hydrogen content of reformable fuel delivered to the anodeand/or a reforming stage associated with the anode relative to thehydrogen reacted in the anode due to the fuel cell anode reaction. Assuch, the “reformable hydrogen surplus ratio” can be computed as(RFC(reformable_anode_in)/(RFC(reformable_anode_in)−RFC(anode_out)),where RFC(reformable_anode_in) refers to the reformable hydrogen contentof reformable fuels in the anode inlet streams or flows, while RFC(anode_out) refers to the reformable hydrogen content of the fuelcomponents (such as H₂, CH₄, and/or CO) in the anode inlet and outletstreams or flows. The RFC can be expressed in moles/s, moles/hr, orsimilar. An example of a method for operating a fuel cell with a largeratio of reformable fuel delivered to the anode reforming stage(s)versus amount of oxidation in the anode can be a method where excessreforming is performed in order to balance the generation andconsumption of heat in the fuel cell. Reforming a reformable fuel toform H₂ and CO is an endothermic process. This endothermic reaction canbe countered by the generation of electrical current in the fuel cell,which can also produce excess heat corresponding (roughly) to thedifference between the amount of heat generated by the anode oxidationreaction and the cathode reaction and the energy that exits the fuelcell in the form of electric current. The excess heat per mole ofhydrogen involved in the anode oxidation reaction/cathode reaction canbe greater than the heat absorbed to generate a mole of hydrogen byreforming. As a result, a fuel cell operated under conventionalconditions can exhibit a temperature increase from inlet to outlet.Instead of this type of conventional operation, the amount of fuelreformed in the reforming stages associated with the anode can beincreased. For example, additional fuel can be reformed so that the heatgenerated by the exothermic fuel cell reactions can be (roughly)balanced by the heat consumed in reforming, or even the heat consumed byreforming can be beyond the excess heat generated by the fuel oxidation,resulting in a temperature drop across the fuel cell. This can result ina substantial excess of hydrogen relative to the amount needed forelectrical power generation. As one example, a feed to the anode inletof a fuel cell or an associated reforming stage can be substantiallycomposed of reformable fuel, such as a substantially pure methane feed.During conventional operation for electric power generation using such afuel, a solid oxide fuel cell can be operated with a fuel utilization ofabout 75%. This means that about 75% (or ¾) of the fuel contentdelivered to the anode is used to form hydrogen that is then reacted inthe anode with oxygen ions to form H₂O. In conventional operation, theremaining about 25% of the fuel content can be reformed to H₂ within thefuel cell (or can pass through the fuel cell unreacted for any CO or H₂in the fuel), and then combusted outside of the fuel cell to form H₂O toprovide heat for the cathode inlet to the fuel cell. The reformablehydrogen surplus ratio in this situation can be 4/(4−1)=4/3.

Electrical efficiency: As used herein, the term “electrical efficiency”(“EE”) is defined as the electrochemical power produced by the fuel celldivided by the rate of Lower Heating Value (“LHV”) of fuel input to thefuel cell. The fuel inputs to the fuel cell includes both fuel deliveredto the anode as well as any fuel used to maintain the temperature of thefuel cell, such as fuel delivered to a burner associated with a fuelcell. In this description, the power produced by the fuel may bedescribed in terms of LHV(el) fuel rate.

Electrochemical power: As used herein, the term “electrochemical power”or LHV(el) is the power generated by the circuit connecting the cathodeto the anode in the fuel cell and the transfer of oxygen ions across thefuel cell's electrolyte. Electrochemical power excludes power producedor consumed by equipment upstream or downstream from the fuel cell. Forexample, electricity produced from heat in a fuel cell exhaust stream isnot considered part of the electrochemical power. Similarly, powergenerated by a gas turbine or other equipment upstream of the fuel cellis not part of the electrochemical power generated. The “electrochemicalpower” does not take electrical power consumed during operation of thefuel cell into account, or any loss incurred by conversion of the directcurrent to alternating current. In other words, electrical power used tosupply the fuel cell operation or otherwise operate the fuel cell is notsubtracted from the direct current power produced by the fuel cell. Asused herein, the power density is the current density multiplied byvoltage. As used herein, the total fuel cell power is the power densitymultiplied by the fuel cell area.

Fuel inputs: As used herein, the term “anode fuel input,” designated asLHV(anode_in), is the amount of fuel within the anode inlet stream. Theterm “fuel input”, designated as LHV(in), is the total amount of fueldelivered to the fuel cell, including both the amount of fuel within theanode inlet stream and the amount of fuel used to maintain thetemperature of the fuel cell. The fuel may include both reformable andnonreformable fuels, based on the definition of a reformable fuelprovided herein. Fuel input is not the same as fuel utilization.

Total fuel cell efficiency: As used herein, the term “total fuel cellefficiency” (“TFCE”) is defined as: the electrochemical power generatedby the fuel cell, plus the rate of LHV of syngas produced by the fuelcell, divided by the rate of LHV of fuel input to the anode. In otherwords, TFCE=(LHV(el)+LHV(sg net))/LHV(anode_in), where LHV(anode_in)refers to rate at which the LHV of the fuel components (such as H₂, CH₄,and/or CO) delivered to the anode and LHV(sg net) refers to a rate atwhich syngas (H₂, CO) is produced in the anode, which is the differencebetween syngas input to the anode and syngas output from the anode.LHV(el) describes the electrochemical power generation of the fuel cell.The total fuel cell efficiency excludes heat generated by the fuel cellthat is put to beneficial use outside of the fuel cell. In operation,heat generated by the fuel cell may be put to beneficial use bydownstream equipment. For example, the heat may be used to generateadditional electricity or to heat water. These uses, when they occurapart from the fuel cell, are not part of the total fuel cellefficiency, as the term is used in this application. The total fuel cellefficiency is for the fuel cell operation only, and does not includepower production, or consumption, upstream, or downstream, of the fuelcell.

Chemical efficiency: As used herein, the term “chemical efficiency”, isdefined as the lower heating value of H₂ and CO in the anode exhaust ofthe fuel cell, or LHV(sg out), divided by the fuel input, or LHV(in).

Neither the electrical efficiency nor the total system efficiency takesthe efficiency of upstream or downstream processes into consideration.For example, it may be advantageous to use turbine exhaust as a sourceof O₂ for the fuel cell cathode. In this arrangement, the efficiency ofthe turbine is not considered as part of the electrical efficiency orthe total fuel cell efficiency calculation. Similarly, outputs from thefuel cell may be recycled as inputs to the fuel cell. A recycle loop isnot considered when calculating electrical efficiency or the total fuelcell efficiency in single pass mode.

Syngas produced: As used herein, the term “syngas produced” is thedifference between syngas input to the anode and syngas output from theanode. Syngas may be used as an input, or fuel, for the anode, at leastin part. For example, a system may include an anode recycle loop thatreturns syngas from the anode exhaust to the anode inlet where it issupplemented with natural gas or other suitable fuel. Syngas producedLHV (sg net)=(LHV(sg out)−LHV(sg in)), where LHV(sg in) and LHV(sg out)refer to the LHV of the syngas in the anode inlet and syngas in theanode outlet streams or flows, respectively. It is noted that at least aportion of the syngas produced by the reforming reactions within ananode can typically be utilized in the anode to produce electricity. Thehydrogen utilized to produce electricity is not included in thedefinition of “syngas produced” because it does not exit the anode. Asused herein, the term “syngas ratio” is the LHV of the net syngasproduced divided by the LHV of the fuel input to the anode or LHV (sgnet)/LHV(anode in). Molar flow rates of syngas and fuel can be usedinstead of LHV to express a molar-based syngas ratio and a molar-basedsyngas produced.

Steam to carbon ratio (S/C): As used herein, the steam to carbon ratio(S/C) is the molar ratio of steam in a flow to reformable carbon in theflow. Carbon in the form of CO and CO₂ are not included as reformablecarbon in this definition. The steam to carbon ratio can be measuredand/or controlled at different points in the system. For example, thecomposition of an anode inlet stream can be manipulated to achieve a S/Cthat is suitable for reforming in the anode. The S/C can be given as themolar flow rate of H₂O divided by the product of the molar flow rate offuel multiplied by the number of carbon atoms in the fuel, e.g., one formethane. Thus, S/C=f_(H2O)/(f_(CH4)×#C), where f_(H2O) is the molar flowrate of water, where f_(CH4) is the molar flow rate of methane (or otherfuel) and #C is the number of carbons in the fuel. In various aspects,the S/C can be about 2, or about 1 to 3, or about 0.5 to 5. It can bedesirable to provide only enough steam to satisfy the reformingreaction's stoichiometry and prevent fouling, as excess steam dilutesthe anode reactants and costs energy to produce.

Fuel Cell and Fuel Cell Components: In this discussion, a fuel cell cancorrespond to a single cell, with an anode and a cathode separated by anelectrolyte. Solid Oxide Fuel Cells can take either a planar form ortubular form. As used herein, a fuel cell can refer to either or bothforms. The anode and cathode can receive input gas flows to facilitatethe respective anode and cathode reactions for transporting chargeacross the electrolyte and generating electricity. A fuel cell stack canrepresent a plurality of cells in an integrated unit. Although a fuelcell stack can include multiple fuel cells, the fuel cells can typicallybe connected in parallel and can function (approximately) as if theycollectively represented a single fuel cell of a larger size. When aninput flow is delivered to the anode or cathode of a fuel cell stack,the fuel stack can include flow channels for dividing the input flowbetween each of the cells in the stack and flow channels for combiningthe output flows from the individual cells. In this discussion, a fuelcell array can be used to refer to a plurality of fuel cells (such as aplurality of fuel cell stacks) that are arranged in series, in parallel,or in any other convenient manner (e.g., in a combination of series andparallel). A fuel cell array can include one or more stages of fuelcells and/or fuel cell stacks, where the anode/cathode output from afirst stage may serve as the anode/cathode input for a second stage. Itis noted that the anodes in a fuel cell array do not have to beconnected in the same way as the cathodes in the array. For convenience,the input to the first anode stage of a fuel cell array may be referredto as the anode input for the array, and the input to the first cathodestage of the fuel cell array may be referred to as the cathode input tothe array. Similarly, the output from the final anode/cathode stage maybe referred to as the anode/cathode output from the array.

It should be understood that reference to use of a fuel cell hereintypically denotes a “fuel cell stack” composed of individual fuel cells,and more generally refers to use of one or more fuel cell stacks influid communication. Individual fuel cell elements (plates or cylinders)can typically be “stacked” together in a rectangular array called a“fuel cell stack”. This fuel cell stack can typically take a feed streamand distribute reactants among all of the individual fuel cell elementsand can then collect the products from each of these elements. Whenviewed as a unit, the fuel cell stack in operation can be taken as awhole even though composed of many (often tens or hundreds) ofindividual fuel cell elements. These individual fuel cell elements cantypically have similar voltages (as the reactant and productconcentrations are similar), and the total power output can result fromthe summation of all of the electrical currents in all of the cellelements, when the elements are electrically connected in series. Stackscan also be arranged in a series arrangement to produce high voltages. Aparallel arrangement can boost the current. If a sufficiently largevolume fuel cell stack is available to process a given flow, the systemsand methods described herein can be used with a single solid oxide fuelcell stack. In other aspects of the invention, a plurality of fuel cellstacks may be desirable or needed for a variety of reasons.

For the purposes of this invention, unless otherwise specified, the term“fuel cell” should be understood to also refer to and/or is defined asincluding a reference to a fuel cell stack composed of set of one ormore individual fuel cell elements for which there is a single input andoutput, as that is the manner in which fuel cells are typically employedin practice. Similarly, the term fuel cells (plural), unless otherwisespecified, should be understood to also refer to and/or is defined asincluding a plurality of separate fuel cell stacks. In other words, allreferences within this document, unless specifically noted, can referinterchangeably to the operation of a fuel cell stack as a “fuel cell”.For example, the volume of exhaust generated by a commercial scalecombustion generator may be too large for processing by a fuel cell(e.g., a single stack) of conventional size. In order to process thefull exhaust, a plurality of fuel cells (i.e., two or more separate fuelcells or fuel cell stacks) can be arranged in parallel, so that eachfuel cell can process (roughly) an equal portion of the combustionexhaust. Although multiple fuel cells can be used, each fuel cell cantypically be operated in a generally similar manner, given its (roughly)equal portion of the combustion exhaust.

“Internal reforming” and “external reforming”: A fuel cell or fuel cellstack may include one or more internal reforming sections. As usedherein, the term “internal reforming” refers to fuel reforming occurringwithin the body of a fuel cell, a fuel cell stack, or otherwise within afuel cell assembly. External reforming, which is often used inconjunction with a fuel cell, occurs in a separate piece of equipmentthat is located outside of the fuel cell stack. In other words, the bodyof the external reformer is not in direct physical contact with the bodyof a fuel cell or fuel cell stack. In a typical set up, the output fromthe external reformer can be fed to the anode inlet of a fuel cell.Unless otherwise noted specifically, the reforming described within thisapplication is internal reforming.

Internal reforming may occur within a fuel cell anode. Internalreforming can additionally or alternately occur within an internalreforming element integrated within a fuel cell assembly. The integratedreforming element may be located between fuel cell elements within afuel cell stack. In other words, one of the trays in the stack can be areforming section instead of a fuel cell element. In one aspect, theflow arrangement within a fuel cell stack directs fuel to the internalreforming elements and then into the anode portion of the fuel cells.Thus, from a flow perspective, the internal reforming elements and fuelcell elements can be arranged in series within the fuel cell stack. Asused herein, the term “anode reforming” is fuel reforming that occurswithin an anode. As used herein, the term “internal reforming” isreforming that occurs within an integrated reforming element and not inan anode section.

In some aspects, a reforming stage that is internal to a fuel cellassembly can be considered to be associated with the anode(s) in thefuel cell assembly. In some alternative aspects, for a reforming stagein a fuel cell stack that can be associated with an anode (such asassociated with multiple anodes), a flow path can be available so thatthe output flow from the reforming stage is passed into at least oneanode. This can correspond to having an initial section of a fuel cellplate that is not in contact with the electrolyte and instead servesjust as a reforming catalyst. Another option for an associated reformingstage can be to have a separate integrated reforming stage as one of theelements in a fuel cell stack, where the output from the integratedreforming stage is returned to the input side of one or more of the fuelcells in the fuel cell stack.

From a heat integration standpoint, a characteristic height in a fuelcell stack can be the height of an individual fuel cell stack element.It is noted that the separate reforming stage or a separate endothermicreaction stage could have a different height in the stack than a fuelcell. In such a scenario, the height of a fuel cell element can be usedas the characteristic height. In some aspects, an integrated endothermicreaction stage can be defined as a stage that is heat integrated withone or more fuel cells, so that the integrated endothermic reactionstage can use the heat from the fuel cells as a heat source forreforming. Such an integrated endothermic reaction stage can be definedas being positioned less than 5 times the height of a stack element fromany fuel cells providing heat to the integrated stage. For example, anintegrated endothermic reaction stage (such as a reforming stage) can bepositioned less than 5 times the height of a stack element from any fuelcells that are heat integrated, such as less than 3 times the height ofa stack element. In this discussion, an integrated reforming stage orintegrated endothermic reaction stage that represents an adjacent stackelement to a fuel cell element can be defined as being about one stackelement height or less away from the adjacent fuel cell element.

In some aspects, a separate reforming stage that is heat integrated witha fuel cell element can also correspond to a reforming stage that isassociated with the fuel cell element. In such aspects, an integratedfuel cell element can provide at least a portion of the heat to theassociated reforming stage, and the associated reforming stage canprovide at least a portion of the reforming stage output to theintegrated fuel cell as a fuel stream. In other aspects, a separatereforming stage can be integrated with a fuel cell for heat transferwithout being associated with the fuel cell. In this type of situation,the separate reforming stage can receive heat from the fuel cell, butthe output of the reforming stage is not used as an input to the fuelcell. Instead, the output of such a reforming stage can be used foranother purpose, such as directly adding the output to the anode exhauststream, or for forming a separate output stream from the fuel cellassembly.

More generally, a separate stack element in a fuel cell stack can beused to perform any convenient type of endothermic reaction that cantake advantage of the waste heat provided by integrated fuel cell stackelements. Instead of plates suitable for performing a reforming reactionon a hydrocarbon fuel stream, a separate stack element can have platessuitable for catalyzing another type of endothermic reaction. A manifoldor other arrangement of inlet conduits in the fuel cell stack can beused to provide an appropriate input flow to each stack element. Asimilar manifold or other arrangement of outlet conduits can also beused to withdraw the output flows from each stack element. Optionally,the output flows from an endothermic reaction stage in a stack can bewithdrawn from the fuel cell stack without having the output flow passthrough a fuel cell anode. In such an optional aspect, the products ofthe exothermic reaction will therefore exit from the fuel cell stackwithout passing through a fuel cell anode. Examples of other types ofendothermic reactions that can be performed in stack elements in a fuelcell stack include ethanol dehydration to form ethylene and ethanecracking.

Recycle: As defined herein, recycle of a portion of a fuel cell output(such as an anode exhaust or a stream separated or withdrawn from ananode exhaust) to a fuel cell inlet can correspond to a direct orindirect recycle stream. A direct recycle of a stream to a fuel cellinlet is defined as recycle of the stream without passing through anintermediate process, while an indirect recycle involves recycle afterpassing a stream through one or more intermediate processes. Forexample, if the anode exhaust is passed through a CO₂ separation stageprior to recycle, this is considered an indirect recycle of the anodeexhaust. If a portion of the anode exhaust, such as an H₂ streamwithdrawn from the anode exhaust, is passed into a gasifier forconverting coal into a fuel suitable for introduction into the fuelcell, then that is also considered an indirect recycle.

Anode Inputs and Outputs

In various aspects of the invention, the SOFC array can be fed by a fuelreceived at the anode inlet that comprises, for example, both hydrogenand a hydrocarbon such as methane (or alternatively a hydrocarbonaceousor hydrocarbon-like compound that may contain heteroatoms different fromC and H). Most of the methane (or other hydrocarbonaceous orhydrocarbon-like compound) fed to the anode can typically be freshmethane. In this description, a fresh fuel such as fresh methane refersto a fuel that is not recycled from another fuel cell process. Forexample, methane recycled from the anode outlet stream back to the anodeinlet may not be considered “fresh” methane, and can instead bedescribed as reclaimed methane. The fuel source used can be shared withother components, such as a turbine. The fuel source input can includewater in a proportion to the fuel appropriate for reforming thehydrocarbon (or hydrocarbon-like) compound in the reforming section thatgenerates hydrogen. For example, if methane is the fuel input forreforming to generate H₂, the molar ratio of water to fuel can be fromabout one to one to about ten to one, such as at least about two to one.A ratio of four to one or greater is typical for external reforming, butlower values can be typical for internal reforming. To the degree thatH₂ is a portion of the fuel source, in some optional aspects noadditional water may be needed in the fuel, as the oxidation of H₂ atthe anode can tend to produce H₂O that can be used for reforming thefuel. The fuel source can also optionally contain components incidentalto the fuel source (e.g., a natural gas feed can contain some content ofCO₂ as an additional component). For example, a natural gas feed cancontain CO₂, N₂, and/or other inert (noble) gases as additionalcomponents. Optionally, in some aspects the fuel source may also containCO, such as CO from a recycled portion of the anode exhaust. Anadditional or alternate potential source for CO in the fuel into a fuelcell assembly can be CO generated by steam reforming of a hydrocarbonfuel performed on the fuel prior to entering the fuel cell assembly.

More generally, a variety of types of fuel streams may be suitable foruse as an input stream for the anode of a solid oxide fuel cell. Somefuel streams can correspond to streams containing hydrocarbons and/orhydrocarbon-like compounds that may also include heteroatoms differentfrom C and H. In this discussion, unless otherwise specified, areference to a fuel stream containing hydrocarbons for an SOFC anode isdefined to include fuel streams containing such hydrocarbon-likecompounds. Examples of hydrocarbon (including hydrocarbon-like) fuelstreams include natural gas, streams containing C1-C4 carbon compounds(such as methane or ethane), and streams containing heavier C5+hydrocarbons (including hydrocarbon-like compounds), as well ascombinations thereof. Still other additional or alternate examples ofpotential fuel streams for use in an anode input can include biogas-typestreams, such as methane produced from natural (biological)decomposition of organic material.

In some aspects, a solid oxide fuel cell can be used to process an inputfuel stream, such as a natural gas and/or hydrocarbon stream, with a lowenergy content due to the presence of diluent compounds. For example,some sources of methane and/or natural gas are sources that can includesubstantial amounts of either CO₂ or other inert molecules, such asnitrogen, argon, or helium. Due to the presence of elevated amounts ofCO₂ and/or inerts, the energy content of a fuel stream based on thesource can be reduced. Using a low energy content fuel for a combustionreaction (such as for powering a combustion-powered turbine) can posedifficulties. However, a solid oxide fuel cell can generate power basedon a low energy content fuel source with a reduced or minimal impact onthe efficiency of the fuel cell. The presence of additional gas volumecan require additional heat for raising the temperature of the fuel tothe temperature for reforming and/or the anode reaction. Additionally,due to the equilibrium nature of the water gas shift reaction within afuel cell anode, the presence of additional CO₂ can have an impact onthe relative amounts of H₂ and CO present in the anode output. However,the inert compounds otherwise can have only a minimal direct impact onthe reforming and anode reactions. The amount of CO₂ and/or inertcompounds in a fuel stream for a solid oxide fuel cell, when present,can be at least about 1 vol %, such as at least about 2 vol %, or atleast about 5 vol %, or at least about 10 vol %, or at least about 15vol %, or at least about 20 vol %, or at least about 25 vol %, or atleast about 30 vol %, or at least about 35 vol %, or at least about 40vol %, or at least about 45 vol %, or at least about 50 vol %, or atleast about 75 vol %. Additionally or alternately, the amount of CO₂and/or inert compounds in a fuel stream for a solid oxide fuel cell canbe about 90 vol % or less, such as about 75 vol % or less, or about 60vol % or less, or about 50 vol % or less, or about 40 vol % or less, orabout 35 vol % or less.

Yet other examples of potential sources for an anode input stream cancorrespond to refinery and/or other industrial process output streams.For example, coking is a common process in many refineries forconverting heavier compounds to lower boiling ranges. Coking typicallyproduces an off-gas containing a variety of compounds that are gases atroom temperature, including CO and various C1-C4 hydrocarbons. Thisoff-gas can be used as at least a portion of an anode input stream.Other refinery off-gas streams can additionally or alternately besuitable for inclusion in an anode input stream, such as light ends(C1-C4) generated during cracking or other refinery processes. Stillother suitable refinery streams can additionally or alternately includerefinery streams containing CO or CO₂ that also contain H₂ and/orreformable fuel compounds.

Still other potential sources for an anode input can additionally oralternately include streams with increased water content. For example,an ethanol output stream from an ethanol plant (or another type offermentation process) can include a substantial portion of H₂O prior tofinal distillation. Such H₂O can typically cause only minimal impact onthe operation of a fuel cell. Thus, a fermentation mixture of alcohol(or other fermentation product) and water can be used as at least aportion of an anode input stream.

Biogas, or digester gas, is another additional or alternate potentialsource for an anode input. Biogas may primarily comprise methane and CO₂and is typically produced by the breakdown or digestion of organicmatter. Anaerobic bacteria may be used to digest the organic matter andproduce the biogas. Impurities, such as sulfur-containing compounds, maybe removed from the biogas prior to use as an anode input.

The output stream from an SOFC anode can include H₂O, CO₂, CO, and H₂.Optionally, the anode output stream could also have unreacted fuel (suchas H₂ or CH₄) or inert compounds in the feed as additional outputcomponents. Instead of using this output stream as a fuel source toprovide heat for a reforming reaction or as a combustion fuel forheating the cell, one or more separations can be performed on the anodeoutput stream to separate the CO₂ from the components with potentialvalue as inputs to another process, such as H₂ or CO. The H₂ and/or COcan be used as a syngas for chemical synthesis, as a source of hydrogenfor chemical reaction, and/or as a fuel with reduced greenhouse gasemissions.

In various aspects, the composition of the output stream from the anodecan be impacted by several factors. Factors that can influence the anodeoutput composition can include the composition of the input stream tothe anode, the amount of current generated by the fuel cell, and/or thetemperature at the exit of the anode. The temperature of at the anodeexit can be relevant due to the equilibrium nature of the water gasshift reaction. In a typical anode, at least one of the plates formingthe wall of the anode can be suitable for catalyzing the water gas shiftreaction. As a result, if a) the composition of the anode input streamis known, b) the extent of reforming of reformable fuel in the anodeinput stream is known, and c) the amount of oxygen ions transported fromthe cathode to anode (corresponding to the amount of electrical currentgenerated) is known, the composition of the anode output can bedetermined based on the equilibrium constant for the water gas shiftreaction.K_(eq)=[CO₂][H₂]/[CO][H₂O]

In the above equation, K_(eq) is the equilibrium constant for thereaction at a given temperature and pressure, and [X] is the partialpressure of component X. Based on the water gas shift reaction, it canbe noted that an increased CO₂ concentration in the anode input can tendto result in additional CO formation (at the expense of H₂) while anincreased H₂O concentration can tend to result in additional H₂formation (at the expense of CO).

To determine the composition at the anode output, the composition of theanode input can be used as a starting point. This composition can thenbe modified to reflect the extent of reforming of any reformable fuelsthat can occur within the anode. Such reforming can reduce thehydrocarbon content of the anode input in exchange for increasedhydrogen and CO₂. Next, based on the amount of electrical currentgenerated, the amount of H₂ in the anode input can be reduced inexchange for additional H₂O and CO₂. This composition can then beadjusted based on the equilibrium constant for the water gas shiftreaction to determine the exit concentrations for H₂, CO, CO₂, and H₂O.

In various aspects, the operating temperature of the SOFC can beselected to achieve a desired ratio of for H₂, CO, and CO₂ within asyngas output. The operating temperature may be selected to generate asyngas output with a ratio suitable to be used in an intended process.In an aspect, the operating temperature can range between about 700° C.and 1200° C., for example the operating temperature can be about 800°C., about 900° C., about 1000° C., or about 1100° C.

Optionally, one or more water gas shift reaction stages can be includedafter the anode output to convert CO and H₂O in the anode output intoCO₂ and H₂, if desired. The amount of H₂ present in the anode output canbe increased, for example, by using a water gas shift reactor at lowertemperature to convert H₂O and CO present in the anode output into H₂and CO₂. As the SOFC can operate at between about 700° C. and 1200° C.,it may be particularly desirable to facilitate a water gas shiftreaction as the anode output is cooled for a use in a subsequentprocess. Alternatively, the temperature can be raised and the water-gasshift reaction can be reversed, producing more CO and H₂O from H₂ andCO₂. Water is an expected output of the reaction occurring at the anode,so the anode output can typically have an excess of H₂O relative to theamount of CO present in the anode output. Alternatively, H₂O can beadded to the stream after the anode exit but before the water gas shiftreaction. CO can be present in the anode output due to incomplete carbonconversion during reforming and/or due to the equilibrium balancingreactions between H₂O, CO, H₂, and CO₂ (i.e., the water-gas shiftequilibrium) under either reforming conditions or the conditions presentduring the anode reaction. A water gas shift reactor can be operatedunder conditions to drive the equilibrium further in the direction offorming CO₂ and H₂ at the expense of CO and H₂O. Higher temperatures cantend to favor the formation of CO and H₂O. Thus, one option foroperating the water gas shift reactor can be to expose the anode outputstream to a suitable catalyst, such as a catalyst including iron oxide,zinc oxide, copper on zinc oxide, or the like, at a suitabletemperature, e.g., between about 190° C. to about 210° C. Optionally,the water-gas shift reactor can include two stages for reducing the COconcentration in an anode output stream, with a first higher temperaturestage operated at a temperature from at least about 300° C. to about375° C. and a second lower temperature stage operated at a temperatureof about 225° C. or less, such as from about 180° C. to about 210° C. Inaddition to increasing the amount of H₂ present in the anode output, thewater-gas shift reaction can additionally or alternately increase theamount of CO₂ at the expense of CO. This can exchangedifficult-to-remove carbon monoxide (CO) for carbon dioxide, which canbe more readily removed by condensation (e.g., cryogenic removal),chemical reaction (such as amine removal), and/or other CO₂ removalmethods. Additionally or alternately, it may be desirable to increasethe CO content present in the anode exhaust in order to achieve adesired ratio of H₂ to CO.

After passing through the optional water gas shift reaction stage, theanode output can be passed through one or more separation stages forremoval of water and/or CO₂ from the anode output stream. For example,one or more CO₂ output streams can be formed by performing CO₂separation on the anode output using one or more methods individually orin combination. Such methods can be used to generate CO₂ outputstream(s) having a CO₂ content of 90 vol % or greater, such as at least95% vol % CO₂, or at least 98 vol % CO₂. Such methods can recover aboutat least about 70% of the CO₂ content of the anode output, such as atleast about 80% of the CO₂ content of the anode output, or at leastabout 90%. Alternatively, in some aspects it may be desirable to recoveronly a portion of the CO₂ within an anode output stream, with therecovered portion of CO₂ being about 33% to about 90% of the CO₂ in theanode output, such as at least about 40%, or at least about 50%. Forexample, it may be desirable to retain some CO₂ in the anode output flowso that a desired composition can be achieved in a subsequent water gasshift stage. Suitable separation methods may comprise use of a physicalsolvent (e.g., Selexol™ or Rectisol™); amines or other bases (e.g., MEAor MDEA); refrigeration (e.g., cryogenic separation); pressure swingadsorption; vacuum swing adsorption; and combinations thereof. Acryogenic CO₂ separator can be an example of a suitable separator. Asthe anode output is cooled, the majority of the water in the anodeoutput can be separated out as a condensed (liquid) phase. Furthercooling and/or pressurizing of the water-depleted anode output flow canthen separate high purity CO₂, as the other remaining components in theanode output flow (such as H₂, N₂, CH₄) do not tend to readily formcondensed phases. A cryogenic CO₂ separator can recover between about33% and about 90% of the CO₂ present in a flow, depending on theoperating conditions.

Removal of water from the anode exhaust to form one or more water outputstreams can also be beneficial, whether prior to, during, or afterperforming CO₂ separation. The amount of water in the anode output canvary depending on operating conditions selected. For example, thesteam-to-carbon ratio established at the anode inlet can affect thewater content in the anode exhaust, with high steam-to-carbon ratiostypically resulting in a large amount of water that can pass through theanode unreacted and/or reacted only due to the water gas shiftequilibrium in the anode. Depending on the aspect, the water content inthe anode exhaust can correspond to up to about 30% or more of thevolume in the anode exhaust. Additionally or alternately, the watercontent can be about 80% or less of the volume of the anode exhaust.While such water can be removed by compression and/or cooling withresulting condensation, the removal of this water can require extracompressor power and/or heat exchange surface area and excessive coolingwater. One beneficial way to remove a portion of this excess water canbe based on use of an adsorbent bed that can capture the humidity fromthe moist anode effluent and can then be ‘regenerated’ using dry anodefeed gas, in order to provide additional water for the anode feed.HVAC-style (heating, ventilation, and air conditioning) adsorptionwheels design can be applicable, because anode exhaust and inlet can besimilar in pressure, and minor leakage from one stream to the other canhave minimal impact on the overall process. In embodiments where CO₂removal is performed using a cryogenic process, removal of water priorto or during CO₂ removal may be desirable, including removal bytriethyleneglycol (TEG) system and/or desiccants. By contrast, if anamine wash is used for CO₂ removal, water can be removed from the anodeexhaust downstream from the CO₂ removal stage.

Alternately or in addition to a CO₂ output stream and/or a water outputstream, the anode output can be used to form one or more product streamscontaining a desired chemical or fuel product. Such a product stream orstreams can correspond to a syngas stream, a hydrogen stream, or bothsyngas product and hydrogen product streams. For example, a hydrogenproduct stream containing at least about 70 vol % H₂, such as at leastabout 90 vol % H₂ or at least about 95 vol % H₂, can be formed.Additionally or alternately, a syngas stream containing at least about70 vol % of H₂ and CO combined, such as at least about 90 vol % of H₂and CO can be formed. The one or more product streams can have a gasvolume corresponding to at least about 75% of the combined H₂ and CO gasvolumes in the anode output, such as at least about 85% or at leastabout 90% of the combined H₂ and CO gas volumes. It is noted that therelative amounts of H₂ and CO in the products streams may differ fromthe H₂ to CO ratio in the anode output based on use of water gas shiftreaction stages to convert between the products.

In some aspects, it can be desirable to remove or separate a portion ofthe H₂ present in the anode output. For example, in some aspects the H₂to CO ratio in the anode exhaust can be at least about 3.0:1. Bycontrast, processes that make use of syngas, such as Fischer-Tropschsynthesis, may consume H₂ and CO in a different ratio, such as a ratiothat is closer to 2:1. One alternative can be to use a water gas shiftreaction to modify the content of the anode output to have an H₂ to COratio closer to a desired syngas composition. Another alternative can beto use a membrane separation to remove a portion of the H₂ present inthe anode output to achieve a desired ratio of H₂ and CO, or stillalternately to use a combination of membrane separation and water gasshift reactions. One advantage of using a membrane separation to removeonly a portion of the H₂ in the anode output can be that the desiredseparation can be performed under relatively mild conditions. Since onegoal can be to produce a retentate that still has a substantial H₂content, a permeate of high purity hydrogen can be generated by membraneseparation without requiring severe conditions. For example, rather thanhaving a pressure on the permeate side of the membrane of about 100 kPaaor less (such as ambient pressure), the permeate side can be at anelevated pressure relative to ambient while still having sufficientdriving force to perform the membrane separation. Additionally oralternately, a sweep gas such as methane can be used to provide adriving force for the membrane separation. This can reduce the purity ofthe H₂ permeate stream, but may be advantageous, depending on thedesired use for the permeate stream.

In various aspects of the invention, at least a portion of the anodeexhaust stream (preferably after separation of CO₂ and/or H₂O) can beused as a feed for a process external to the fuel cell and associatedreforming stages. In various aspects, the anode exhaust can have a ratioof H₂ to CO of about 1.5:1 to about 10:1, such as at least about 3.0:1,or at least about 4.0:1, or at least about 5.0:1. A syngas stream can begenerated or withdrawn from the anode exhaust. The anode exhaust gas,optionally after separation of CO₂ and/or H₂O, and optionally afterperforming a water gas shift reaction and/or a membrane separation toremove excess hydrogen, can correspond to a stream containingsubstantial portions of H₂ and/or CO. For a stream with a relatively lowcontent of CO, such as a stream where the ratio of H₂ to CO is at leastabout 3:1, the anode exhaust can be suitable for use as an H₂ feed.Examples of processes that could benefit from an H₂ feed can include,but are not limited to, refinery processes, an ammonia synthesis plant,or a turbine in a (different) power generation system, or combinationsthereof. Depending on the application, still lower CO₂ contents can bedesirable. For a stream with an H₂-to-CO ratio of less than about 2.2 to1 and greater than about 1.9 to 1, the stream can be suitable for use asa syngas feed. Examples of processes that could benefit from a syngasfeed can include, but are not limited to, a gas-to-liquids plant (suchas a plant using a Fischer-Tropsch process with a non-shifting catalyst)and/or a methanol synthesis plant. The amount of the anode exhaust usedas a feed for an external process can be any convenient amount.Optionally, when a portion of the anode exhaust is used as a feed for anexternal process, a second portion of the anode exhaust can be recycledto the anode input and/or recycled to the combustion zone for acombustion-powered generator.

The input streams useful for different types of Fischer-Tropschsynthesis processes can provide an example of the different types ofproduct streams that may be desirable to generate from the anode output.For a Fischer-Tropsch synthesis reaction system that uses a shiftingcatalyst, such as an iron-based catalyst, the desired input stream tothe reaction system can include CO₂ in addition to H₂ and CO. If asufficient amount of CO₂ is not present in the input stream, aFischer-Tropsch catalyst with water gas shift activity can consume CO inorder to generate additional CO₂, resulting in a syngas that can bedeficient in CO. For integration of such a Fischer-Tropsch process withan SOFC fuel cell, the separation stages for the anode output can beoperated to retain a desired amount of CO₂ (and optionally H₂O) in thesyngas product. By contrast, for a Fischer-Tropsch catalyst based on anon-shifting catalyst, any CO₂ present in a product stream could serveas an inert component in the Fischer-Tropsch reaction system.

In an aspect where the membrane is swept with a sweep gas such as amethane sweep gas, the methane sweep gas can correspond to a methanestream used as the anode fuel or in a different low pressure process,such as a boiler, furnace, gas turbine, or other fuel-consuming device.In such an aspect, low levels of CO₂ permeation across the membrane canhave minimal consequence. Such CO₂ that may permeate across the membranecan have a minimal impact on the reactions within the anode, and suchCO₂ can remain contained in the anode product. Therefore, the CO₂ (ifany) lost across the membrane due to permeation does not need to betransferred again across the SOFC electrolyte. This can significantlyreduce the separation selectivity requirement for the hydrogenpermeation membrane. This can allow, for example, use of ahigher-permeability membrane having a lower selectivity, which canenable use of a lower pressure and/or reduced membrane surface area. Insuch an aspect of the invention, the volume of the sweep gas can be alarge multiple of the volume of hydrogen in the anode exhaust, which canallow the effective hydrogen concentration on the permeate side to bemaintained close to zero. The hydrogen thus separated can beincorporated into the turbine-fed methane where it can enhance theturbine combustion characteristics, as described above.

It is noted that excess H₂ produced in the anode can represent a fuelwhere the greenhouse gases have already been separated. Any CO₂ in theanode output can be readily separated from the anode output, such as byusing an amine wash, a cryogenic CO₂ separator, and/or a pressure orvacuum swing absorption process. Several of the components of the anodeoutput (H₂, CO, CH₄) are not easily removed, while CO₂ and H₂O canusually be readily removed. Depending on the embodiment, at least about90 vol % of the CO₂ in the anode output can be separated out to form arelatively high purity CO₂ output stream. Thus, any CO₂ generated in theanode can be efficiently separated out to form a high purity CO₂ outputstream. After separation, the remaining portion of the anode output cancorrespond primarily to components with chemical and/or fuel value, aswell as reduced amounts of CO₂ and/or H₂O. Since a substantial portionof the CO₂ generated by the original fuel (prior to reforming) can havebeen separated out, the amount of CO₂ generated by subsequent burning ofthe remaining portion of the anode output can be reduced. In particular,to the degree that the fuel in the remaining portion of the anode outputis H₂, no additional greenhouse gases can typically be formed by burningof this fuel.

The anode exhaust can be subjected to a variety of gas processingoptions, including water-gas shift and separation of the components fromeach other. Two general anode processing schemes are shown in FIGS. 1and 2.

FIG. 1 schematically shows an example of a reaction system for operatinga fuel cell array of solid oxide fuel cells in conjunction with achemical synthesis process. In FIG. 1, a fuel stream 105 is provided toa reforming stage (or stages) 110 associated with the anode 127 of afuel cell 120, such as a fuel cell that is part of a fuel cell stack ina fuel cell array. The reforming stage 110 associated with fuel cell 120can be internal to a fuel cell assembly. In some optional aspects, anexternal reforming stage (not shown) can also be used to reform aportion of the reformable fuel in an input stream prior to passing theinput stream into a fuel cell assembly. Fuel stream 105 can preferablyinclude a reformable fuel, such as methane, other hydrocarbons, and/orother hydrocarbon-like compounds such as organic compounds containingcarbon-hydrogen bonds. Fuel stream 105 can also optionally contain H₂and/or CO, such as H₂ and/or CO provided by optional anode recyclestream 185. It is noted that anode recycle stream 185 is optional, andthat in many aspects no recycle stream is provided from the anodeexhaust 125 back to anode 127, either directly or indirectly viacombination with fuel stream 105 or reformed fuel stream 115. Afterreforming, the reformed fuel stream 115 can be passed into anode 127 offuel cell 120. An O₂-containing stream 119 can also be passed intocathode 129. A flow of oxygen ions 122, O₂ ²⁻, from the cathode portion129 of the fuel cell can provide the remaining reactant needed for theanode fuel cell reactions. Based on the reactions in the anode 127, theresulting anode exhaust 125 can include H₂O, one or more componentscorresponding to incompletely reacted fuel (H_(z), CO, CH₄, or othercomponents corresponding to a reformable fuel), and optionally one ormore additional nonreactive components, such as CO₂, N₂ and/or othercontaminants that are part of fuel stream 105. The anode exhaust 125 canthen be passed into one or more separation stages. For example, a CO₂removal stage 140 can correspond to a cryogenic CO₂ removal system, anamine wash stage for removal of acid gases such as CO₂, or anothersuitable type of CO₂ separation stage for separating a CO₂ output stream143 from the anode exhaust. Optionally, the anode exhaust can first bepassed through a water gas shift reactor 130 to convert any CO presentin the anode exhaust (along with some H₂O) into CO₂ and H₂ in anoptionally water gas shifted anode exhaust 135. Depending on the natureof the CO₂ removal stage, a water condensation or removal stage 150 maybe desirable to remove a water output stream 153 from the anode exhaust.Though shown in FIG. 1 after the CO₂ separation stage 140, it mayoptionally be located before the CO₂ separation stage 140 instead.Additionally, an optional membrane separation stage 160 for separationof H₂ can be used to generate a high purity permeate stream 163 of H₂.The resulting retentate stream 166 can then be used as an input to achemical synthesis process. Stream 166 could additionally or alternatelybe shifted in a second water-gas shift reactor 131 to adjust the H₂, CO,and CO₂ content to a different ratio, producing an output stream 168 forfurther use in a chemical synthesis process. In FIG. 1, anode recyclestream 185 is shown as being withdrawn from the retentate stream 166,but the anode recycle stream 185 could additionally or alternately bewithdrawn from other convenient locations in or between the variousseparation stages. The separation stages and shift reactor(s) couldadditionally or alternately be configured in different orders, and/or ina parallel configuration. Finally, a stream with a reduced content of O₂139 can be generated as an output from cathode 129. For the sake ofsimplicity, various stages of compression and heat addition/removal thatmight be useful in the process, as well as steam addition or removal,are not shown.

As noted above, the various types of separations performed on the anodeexhaust can be performed in any convenient order. FIG. 2 shows anexample of an alternative order for performing separations on an anodeexhaust. In FIG. 2, anode exhaust 125 can be initially passed intoseparation stage 260 for removing a portion 263 of the hydrogen contentfrom the anode exhaust 125. This can allow, for example, reduction ofthe H₂ content of the anode exhaust to provide a retentate 266 with aratio of H₂ to CO closer to 2:1. The ratio of H₂ to CO can then befurther adjusted to achieve a desired value in a water gas shift stage230. The water gas shifted output 235 can then pass through CO₂separation stage 240 and water removal stage 250 to produce an outputstream 275 suitable for use as an input to a desired chemical synthesisprocess. Optionally, output stream 275 could be exposed to an additionalwater gas shift stage (not shown). A portion of output stream 275 canoptionally be recycled (not shown) to the anode input. Of course, stillother combinations and sequencing of separation stages can be used togenerate a stream based on the anode output that has a desiredcomposition. For the sake of simplicity, various stages of compressionand heat addition/removal that might be useful in the process, as wellas steam addition or removal, are not shown.

Cathode Inputs and Outputs

Conventionally, a solid oxide fuel cell can be operated based on drawinga desired load while consuming some portion of the fuel in the fuelstream delivered to the anode. The voltage of the fuel cell can then bedetermined by the load, fuel input to the anode, air and O₂ provided tothe cathode, and the internal resistances of the fuel cell. By removingany direct link between the composition of the anode input flow and thecathode input flow, additional options become available for operatingthe fuel cell, such as to generate excess synthesis gas and/or toimprove the total efficiency (electrical plus chemical power) of thefuel cell, among others.

The amount of O₂ present in a cathode input stream can advantageously besufficient to provide the oxygen needed for the cathode reaction in thefuel cell. Thus, the volume percentage of O₂ can advantageously be atleast 0.5 times the amount of O₂ in the cathode exhaust. Optionally, asnecessary, additional air can be added to the cathode input to providesufficient oxidant for the cathode reaction. When some form of air isused as the oxidant, the amount of N₂ in the cathode exhaust can be atleast about 78 vol %, e.g., at least about 88 vol %, and/or about 95 vol% or less. In some aspects, the cathode input stream can additionally oralternately contain compounds that are generally viewed as contaminants,such as H₂S or NH₃. In other aspects, the cathode input stream can becleaned to reduce or minimize the content of such contaminants.

The conditions in the cathode can additionally or alternately besuitable for conversion of unburned hydrocarbons (in combination with O₂in the cathode input stream) to typical combustion products, such as CO₂and H₂O.

Fuel Cell Arrangement

In various aspects, a fuel cell array can be operated to improve ormaximize the energy output of the fuel cell, such as the total energyoutput, the electric energy output, the syngas chemical energy output,or a combination thereof. For example, solid oxide fuel cells can beoperated with an excess of reformable fuel in a variety of situations,such as for generation of a syngas stream for use in chemical synthesisplant and/or for generation of a high purity hydrogen stream. The syngasstream and/or hydrogen stream can be used as a syngas source, a hydrogensource, as a clean fuel source, and/or for any other convenientapplication. In such aspects, the amount of O₂ in the cathode exhaustcan be related to the amount of O₂ in the cathode input stream and theO₂ utilization at the desired operating conditions for improving ormaximizing the fuel cell energy output.

Solid Oxide Fuel Cell Operation

In an aspect, the operating temperature of the SOFC can range betweenabout 700 C and 1200 C, for example the operating temperature can beabout 800 C, about 900 C, about 1000 C, or about 1100 C. In an aspect,the operating temperature may be selected to push the WGS reactionwithin the anode to a desired ratio.

In some aspects, a fuel cell may be operated in a single pass oronce-through mode. In single pass mode, reformed products in the anodeexhaust are not returned to the anode inlet. Thus, recycling syngas,hydrogen, or some other product from the anode output directly to theanode inlet is not done in single pass operation. More generally, insingle pass operation, reformed products in the anode exhaust are alsonot returned indirectly to the anode inlet, such as by using reformedproducts to process a fuel stream subsequently introduced into the anodeinlet. Heat from the anode exhaust or output may additionally oralternately be recycled in a single pass mode. For example, the anodeoutput flow may pass through a heat exchanger that cools the anodeoutput and warms another stream, such as an input stream for the anodeand/or the cathode. Recycling heat from anode to the fuel cell isconsistent with use in single pass or once-through operation. Optionallybut not preferably, constituents of the anode output may be burned toprovide heat to the fuel cell during single pass mode.

FIG. 3 shows a schematic example of the operation of an SOFC forgeneration of electrical power. In FIG. 3, the anode portion of the fuelcell can receive fuel and steam (H₂O) as inputs, with outputs of water,and optionally excess H₂, CH₄ (or other hydrocarbons), and/or CO. Thecathode portion of the fuel cell can receive O₂ (e.g., air) as inputs,with an output corresponding to O₂-depleted oxidant (air). Within thefuel cell, O₂ ²⁻ ions formed in the cathode side can be transportedacross the electrolyte to provide the oxygen ions needed for thereactions occurring at the anode.

Several reactions can occur within a solid oxide fuel cell such as theexample fuel cell shown in FIG. 3. The reforming reactions can beoptional, and can be reduced or eliminated if sufficient H₂ is provideddirectly to the anode. The following reactions are based on CH₄, butsimilar reactions can occur when other fuels are used in the fuel cell.<anode reforming> CH₄+H₂O=>3H₂+CO  (1)<water gas shift> CO+H₂O=>H₂+CO₂  (2)<reforming and water gas shift combined> CH₄+2H₂O=>4H₂+CO₂  (3)<anode H₂ oxidation> H₂+O₂ ²⁻=>H₂O+2e ⁻  (4)<cathode> ½O₂+2e ⁻=>O₂ ²⁻  (5)

Reaction (1) represents the basic hydrocarbon reforming reaction togenerate H₂ for use in the anode of the fuel cell. The CO formed inreaction (1) can be converted to H₂ by the water-gas shift reaction (2).The combination of reactions (1) and (2) is shown as reaction (3).Reactions (1) and (2) can occur external to the fuel cell, and/or thereforming can be performed internal to the anode.

Reactions (4) and (5), at the anode and cathode respectively, representthe reactions that can result in electrical power generation within thefuel cell. Reaction (4) combines H₂, either present in the feed oroptionally generated by reactions (1) and/or (2), with oxygen ions toform H₂O and electrons to the circuit. Reaction (5) combines O₂ andelectrons from the circuit to form oxygen ions. The oxygen ionsgenerated by reaction (5) can be transported across the electrolyte ofthe fuel cell to provide the oxygen ions needed for reaction (4). Incombination with the transport of oxygen ions across the electrolyte, aclosed current loop can then be formed by providing an electricalconnection between the anode and cathode.

In various embodiments, a goal of operating the fuel cell can be toimprove the total efficiency of the fuel cell and/or the totalefficiency of the fuel cell plus an integrated chemical synthesisprocess. This is typically in contrast to conventional operation of afuel cell, where the goal can be to operate the fuel cell with highelectrical efficiency for using the fuel provided to the cell forgeneration of electrical power. As defined above, total fuel cellefficiency may be determined by dividing the electric output of the fuelcell plus the lower heating value of the fuel cell outputs by the lowerheating value of the input components for the fuel cell. In other words,TFCE=(LHV(el)+LHV(sg out))/LHV(in), where LHV(in) and LHV(sg out) referto the LHV of the fuel components (such as H₂, CH₄, and/or CO) deliveredto the fuel cell and syngas (H₂, CO and/or CO₂) in the anode outletstreams or flows, respectively. This can provide a measure of theelectric energy plus chemical energy generated by the fuel cell and/orthe integrated chemical process. It is noted that under this definitionof total efficiency, heat energy used within the fuel cell and/or usedwithin the integrated fuel cell/chemical synthesis system can contributeto total efficiency. However, any excess heat exchanged or otherwisewithdrawn from the fuel cell or integrated fuel cell/chemical synthesissystem is excluded from the definition. Thus, if excess heat from thefuel cell is used, for example, to generate steam for electricitygeneration by a steam turbine, such excess heat is excluded from thedefinition of total efficiency.

Several operational parameters may be manipulated to operate a fuel cellwith excess reformable fuel. Some parameters can be similar to thosecurrently recommended for fuel cell operation. In some aspects, thecathode conditions and temperature inputs to the fuel cell can besimilar to those recommended in the literature. For example, the desiredelectrical efficiency and the desired total fuel cell efficiency may beachieved at a range of fuel cell operating temperatures typical forsolid oxide fuel cells. In typical operation, the temperature canincrease across the fuel cell.

In other aspects, the operational parameters of the fuel cell candeviate from typical conditions so that the fuel cell is operated toallow a temperature decrease from the anode inlet to the anode outletand/or from the cathode inlet to the cathode outlet. For example, thereforming reaction to convert a hydrocarbon into H₂ and CO is anendothermic reaction. If a sufficient amount of reforming is performedin a fuel cell anode relative to the amount of oxidation of hydrogen togenerate electrical current, the net heat balance in the fuel cell canbe endothermic. This can cause a temperature drop between the inlets andoutlets of a fuel cell. During endothermic operation, the temperaturedrop in the fuel cell can be controlled so that the electrolyte in thefuel cell remains in a molten state.

Parameters that can be manipulated in a way so as to differ from thosecurrently recommended can include the amount of fuel provided to theanode, the composition of the fuel provided to the anode, and/or theseparation and capture of syngas in the anode output without significantrecycling of syngas from the anode exhaust to either the anode input orthe cathode input. In some aspects, no recycle of syngas or hydrogenfrom the anode exhaust to either the anode input or the cathode inputcan be allowed to occur, either directly or indirectly. In additional oralternative aspects, a limited amount of recycle can occur. In suchaspects, the amount of recycle from the anode exhaust to the anode inputand/or the cathode input can be less than about 10 vol % of the anodeexhaust, such as less than about 5 vol %, or less than about 1 vol %.

In some embodiments, the fuel cells in a fuel cell array can be arrangedso that only a single stage of fuel cells (such as fuel cell stacks) canbe present. In this type of embodiment, the anode fuel utilization forthe single stage can represent the anode fuel utilization for the array.Another option can be that a fuel cell array can contain multiple stagesof anodes and multiple stages of cathodes, with each anode stage havinga fuel utilization within the same range, such as each anode stagehaving a fuel utilization within 10% of a specified value, for examplewithin 5% of a specified value. Still another option can be that eachanode stage can have a fuel utilization equal to a specified value orlower than the specified value by less than an amount, such as havingeach anode stage be not greater than a specified value by 10% or less,for example, by 5% or less. As an illustrative example, a fuel cellarray with a plurality of anode stages can have each anode stage bewithin about 10% of 50% fuel utilization, which would correspond to eachanode stage having a fuel utilization between about 40% and about 60%.As another example, a fuel cell array with a plurality of stages canhave each anode stage be not greater than 60% anode fuel utilizationwith the maximum deviation being about 5% less, which would correspondto each anode stage having a fuel utilization between about 55% to about60%. In still another example, one or more stages of fuel cells in afuel cell array can be operated at a fuel utilization from about 30% toabout 50%, such as operating a plurality of fuel cell stages in thearray at a fuel utilization from about 30% to about 50%. More generally,any of the above types of ranges can be paired with any of the anodefuel utilization values specified herein.

Yet still another additional or alternate option can be to specify anoverall average of fuel utilization over all fuel cells in a fuel cellarray. In various aspects, the overall average of fuel utilization for afuel cell array can be about 65% or less, for example, about 60% orless, about 55% or less, about 50% or less, or about 45% or less(additionally or alternately, the overall average fuel utilization for afuel cell array can be at least about 25%, for example at least about30%, at least about 35%, or at least about 40%). Such an average fuelutilization need not necessarily constrain the fuel utilization in anysingle stage, so long as the array of fuel cells meets the desired fuelutilization.

Applications for Syngas Output after Capture

The components from an anode output stream and/or cathode output streamcan be used for a variety of purposes. One option can be to use theanode output as a source of hydrogen, as described above. For an SOFCintegrated with or co-located with a refinery, the hydrogen can be usedas a hydrogen source for various refinery processes, such ashydroprocessing. Such hydrogen can be used in a refinery or otherindustrial setting as a fuel for a boiler, furnace, and/or fired heater,and/or the hydrogen can be used as a feed for an electric powergenerator, such as a turbine. Hydrogen from an SOFC fuel cell canfurther additionally or alternately be used as an input stream for othertypes of fuel cells that require hydrogen as an input, possiblyincluding vehicles powered by fuel cells. Still another option can be toadditionally or alternately use syngas generated as an output from anSOFC fuel cell as a fermentation input.

Another option can be to additionally or alternately use syngasgenerated from the anode output. Of course, syngas can be used as afuel, although a syngas based fuel can still lead to some CO₂ productionwhen burned as fuel. In other aspects, a syngas output stream can beused as an input for a chemical synthesis process. One option can be toadditionally or alternately use syngas for a Fischer-Tropsch typeprocess, and/or another process where larger hydrocarbon molecules areformed from the syngas input. Another option can be to additionally oralternately use syngas to form an intermediate product such as methanol.Methanol could be used as the final product, but in other aspectsmethanol generated from syngas can be used to generate larger compounds,such as gasoline, olefins, aromatics, and/or other products. It is notedthat a small amount of CO₂ can be acceptable in the syngas feed to amethanol synthesis process, and/or to a Fischer-Tropsch processutilizing a shifting catalyst. Hydroformylation is an additional oralternate example of still another synthesis process that can make useof a syngas input.

It is noted that one variation on use of an SOFC to generate syngas canbe to use SOFC fuel cells as part of a system for processing methaneand/or natural gas withdrawn by an offshore oil platform or otherproduction system that is a considerable distance from its ultimatemarket. Instead of attempting to transport the gas phase output from awell, or attempting to store the gas phase product for an extendedperiod, the gas phase output from a well can be used as the input to anSOFC fuel cell array. This can lead to a variety of benefits. First, theelectric power generated by the fuel cell array can be used as a powersource for the platform. Additionally, the syngas output from the fuelcell array can be used as an input for a Fischer-Tropsch process at theproduction site. This can allow for formation of liquid hydrocarbonproducts more easily transported by pipeline, ship, or railcar from theproduction site to, for example, an on-shore facility or a largerterminal.

Still other integration options can additionally or alternately includeusing the cathode output as a source of higher purity, heated nitrogen.The cathode input can often include a large portion of air, which meansa substantial portion of nitrogen can be included in the cathode input.The fuel cell can transport O₂ from the cathode across the electrolyteto the anode, and the cathode outlet can have lower concentrations ofO₂, and thus a higher concentration of N₂ than found in air. Withsubsequent removal of the residual O₂, this nitrogen output can be usedas an input for production of ammonia or other nitrogen-containingchemicals, such as urea, ammonium nitrate, and/or nitric acid. It isnoted that urea synthesis could additionally or alternately use CO₂separate from the anode output as an input feed.

Additional Embodiments Embodiment 1

A method for producing electricity, and hydrogen or syngas, using asolid oxide fuel cell having an anode and cathode, the methodcomprising: introducing a fuel stream comprising a reformable fuel intothe anode of the solid oxide fuel cell, a reforming stage (comprising aninternal reforming element) associated with the anode of the solid oxidefuel cell, or a combination thereof; introducing a cathode inlet streamcomprising O₂ into the cathode of the solid oxide fuel cell; generatingelectricity within the solid oxide fuel cell; and withdrawing, from ananode exhaust, a gas stream comprising H₂, a gas stream comprising H₂and CO, or a combination thereof, wherein an electrical efficiency forthe solid oxide fuel cell is between about 10% and about 50% and a totalfuel cell productivity for the solid oxide fuel cell is at least about150 mW/cm².

Embodiment 2

The method of Embodiment 1, wherein the solid oxide fuel cell isoperated to generate electricity at a thermal ratio of about 0.25 toabout 1.3, or about 1.15 or less, or about 1.0 or less, or about 0.75 orless.

Embodiment 3

The method of any of the above embodiments, wherein a reformable fuelsurplus ratio of the fuel stream comprising the reformable fuel is atleast about 2.0, or at least about 2.5.

Embodiment 4

The method of any of the above embodiments, wherein the electricalefficiency of the solid oxide fuel cell is about 45% or less, or about35% or less.

Embodiment 5

The method of any of the above embodiments, wherein a total fuel cellefficiency for the solid oxide fuel cell is at least about 65%, or atleast about 70%, or at least about 75%, or at least about 80%.

Embodiment 6

The method of any of the above embodiments, wherein the total fuel cellproductivity for the solid oxide fuel cell is at least about 150 mW/cm²,or at least about 300 mW/cm², or at least about 350 mW/cm², or about 800mW/cm² or less.

Embodiment 7

The method of any of the above embodiments, wherein a total reformablefuel productivity for the solid oxide fuel cell is at least about 75mW/cm², or at least about 100 mW/cm², or at least about 150 mW/cm², orat least about 200 mW/cm², or about 600 mW/cm² or less.

Embodiment 8

The method of any of the above embodiments, wherein a reformablehydrogen content of reformable fuel introduced into the anode of thesolid oxide fuel cell, a reforming stage (comprising an internalreforming element) associated with the anode of the solid oxide fuelcell, or a combination thereof is at least about 75% greater than theamount of hydrogen reacted to generate electricity, such as at leastabout 100% greater.

Embodiment 9

The method of any of the above embodiments, wherein the fuel streamcomprises at least about 10 vol % inert compounds, at least about 10 vol% CO₂, or a combination thereof.

Embodiment 10

The method of any of the above embodiments, wherein the fuel cell isoperated at a voltage V_(A) of about 0.67 Volts or less, or about 0.5Volts or less.

Embodiment 11

The method of any of the above embodiments, wherein the anode exhausthas a ratio of H₂ to CO of about 1.5:1 to about 10:1.

Embodiment 12

The method of any of the above embodiments, wherein the anode exhausthas a ratio of H₂ to CO of at least about 3.0:1.

Embodiment 13

The method of any of the above embodiments, wherein the solid oxide fuelcell is a tubular solid oxide fuel cell.

Embodiment 14

The method of any of the above embodiments, wherein the solid oxide fuelcell further comprises one or more integrated endothermic reactionstages.

Embodiment 15

The method of Embodiment 14, wherein at least one integrated endothermicreaction stage comprises an integrated reforming stage, the fuel streamintroduced into the anode of the solid oxide fuel cell being passedthrough the integrated reforming stage prior to entering the anode ofthe solid oxide fuel cell.

Embodiment 16

The method of any of Embodiments 1-15, wherein a temperature at theanode outlet is greater than a temperature at the anode inlet by about40° C. or less.

Embodiment 17

The method of any of Embodiments 1-15, wherein a temperature at theanode inlet differs from a temperature at the anode outlet by about 20°C. or less.

Embodiment 18

The method of any of Embodiments 1-15, wherein a temperature at theanode outlet is less than a temperature at the anode inlet by about 10°C. to about 80° C.

Embodiment 19

The method of any of Embodiments 1-15, wherein a thermal ratio is about0.85 or less, the method further comprising supplying heat to the fuelcell to maintain a temperature at the anode outlet that is less than thetemperature at the anode inlet by about 5° C. to about 50° C.

Embodiment 20

The method of any of the above embodiments, the method furthercomprising reforming the reformable fuel, wherein at least about 90% ofthe reformable fuel introduced into the anode of the solid oxide fuelcell, a reforming stage (comprising an internal reforming element)associated with the anode of the solid oxide fuel cell, or a combinationthereof is reformed in a single pass through the anode of the solidoxide fuel cell.

Although the present invention has been described in terms of specificembodiments, it is not so limited. Suitable alterations/modificationsfor operation under specific conditions should be apparent to thoseskilled in the art. It is therefore intended that the following claimsbe interpreted as covering all such alterations/modifications that fallwithin the true spirit/scope of the invention.

What is claimed is:
 1. A method for producing electricity, and hydrogenor syngas, using a solid oxide fuel cell having an anode and cathode,the method comprising: introducing a fuel stream comprising a reformablefuel having a reformable fuel surplus ratio of at least about 2.0 intothe anode of the solid oxide fuel cell, a reforming stage associatedwith the anode of the solid oxide fuel cell, or a combination thereof;introducing a cathode inlet stream comprising O2 into the cathode of thesolid oxide fuel cell; generating electricity within the solid oxidefuel cell; and withdrawing, from an anode exhaust, a gas streamcomprising Hz, a gas stream comprising H2 and CO, or a combinationthereof, wherein an electrical efficiency for the solid oxide fuel cellis between about 10% and about 50%, a total reformable fuel productivityfor the solid oxide fuel cell is at least about 75 mW/cm2 and a totalfuel cell productivity for the solid oxide fuel cell is at least about150 mW/cm2.
 2. The method of claim 1, wherein the solid oxide fuel cellis operated to generate electricity at a thermal ratio of about 1.3 orless.
 3. The method of claim 2, wherein the thermal ratio is about 0.85or less, the method further comprising supplying heat to the fuel cellto maintain a temperature at the anode outlet that is less than thetemperature at the anode inlet by about 5° C. to about 50° C.
 4. Themethod of claim 1, wherein the electrical efficiency of the solid oxidefuel cell is about 35% or less.
 5. The method of claim 1, wherein atotal fuel cell efficiency for the solid oxide fuel cell is at leastabout 70%.
 6. The method of claim 1, wherein the total fuel cellproductivity for the solid oxide fuel cell is at least about 250 mW/cm2.7. The method of claim 1, wherein a reformable hydrogen content ofreformable fuel introduced into the anode of the solid oxide fuel cell,a reforming stage associated with the anode of the solid oxide fuelcell, or a combination thereof is at least about 75% greater than theamount of hydrogen reacted to generate electricity.
 8. The method ofclaim 1, wherein the fuel stream comprises at least about 10 vol % inertcompounds, at least about 10 vol % CO2, or a combination thereof.
 9. Themethod of claim 1, wherein the fuel cell is operated at a voltage VA ofabout 0.67 Volts or less.
 10. The method of claim 1, wherein the anodeexhaust has a ratio of H2 to CO of about 1.5:1 to about 10:1.
 11. Themethod of claim 1, wherein the anode exhaust has a ratio of H2 to CO ofat least about 2.0:1.
 12. The method of claim 1, wherein the solid oxidefuel cell is a tubular solid oxide fuel cell.
 13. The method of claim 1,wherein the solid oxide fuel cell further comprises one or moreintegrated endothermic reaction stages.
 14. The method of claim 13,wherein at least one integrated endothermic reaction stage comprises anintegrated reforming stage, the fuel stream introduced into the anode ofthe solid oxide fuel cell being passed through the integrated reformingstage prior to entering the anode of the solid oxide fuel cell.
 15. Themethod of claim 1, wherein a temperature at the anode outlet is greaterthan a temperature at the anode inlet by about 40° C. or less.
 16. Themethod of claim 1, wherein a temperature at the anode inlet differs froma temperature at the anode outlet by about 20° C. or less.
 17. Themethod of claim 1, wherein a temperature at the anode outlet is lessthan a temperature at the anode inlet by about 10° C. to about 80° C.18. The method of claim 1, the method further comprising reforming thereformable fuel, wherein at least about 90% of the reformable fuelintroduced into the anode of the solid oxide fuel cell, a reformingstage associated with the anode of the solid oxide fuel cell, or acombination thereof is reformed in a single pass through the anode ofthe solid oxide fuel cell.